Annual Report of Employee Stock Purchase/Savings Plan (Form 11-K)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 11-K

(Mark One)

For the fiscal year ended December 31, 2025

or

For the transition period from to

Commission file number 1-15399

Packaging Corporation of America

Thrift Plan for Hourly Employees

Packaging Corporation of America

1 North Field Court

Lake Forest, IL 60045

Packaging Corporation of America

Thrift Plan for Hourly Employees

* Schedules required by Form 5500 that are not applicable have not been included.

2

Report of Independent Registered Public Accounting Firm

Plan Administrator and Plan Participants

Packaging Corporation of America Thrift Plan for Hourly Employees

Opinion on the financial statements

We have audited the accompanying statements of net assets available for benefits of Packaging Corporation of America Thrift Plan for Hourly Employees (the "Plan") as of December 31, 2025 and 2024, the related statement of changes in net assets available for benefits for the year ended December 31, 2025, and the related notes (collectively referred to as the "financial statements"). In our opinion, the financial statements present fairly, in all material respects, the net assets available for benefits of the Plan as of December 31, 2025 and 2024, and the changes in net assets available for benefits for the year ended December 31, 2025 in conformity with accounting principles generally accepted in the United States of America.

Basis for opinion

These financial statements are the responsibility of the Plan's management. Our responsibility is to express an opinion on the Plan's financial statements based on our audits. We are a public accounting firm registered with the Public Company Accounting Oversight Board (United States) ("PCAOB") and are required to be independent with respect to the Plan in accordance with the U.S. federal securities laws and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.

We conducted our audits in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement, whether due to error or fraud. The Plan is not required to have, nor were we engaged to perform, an audit of its internal control over financial reporting. As part of our audits we are required to obtain an understanding of internal control over financial reporting but not for the purpose of expressing an opinion on the effectiveness of the Plan's internal control over financial reporting. Accordingly, we express no such opinion.

Our audits included performing procedures to assess the risks of material misstatement of the financial statements, whether due to error or fraud, and performing procedures that respond to those risks. Such procedures included examining, on a test basis, evidence regarding the amounts and disclosures in the financial statements. Our audits also included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the financial statements. We believe that our audits provide a reasonable basis for our opinion.

Supplemental information

The schedule of assets (held at end of year) as of December 31, 2025 ("supplemental information") has been subjected to audit procedures performed in conjunction with the audit of the Plan's financial statements. The supplemental information is the responsibility of the Plan's management. Our audit procedures included determining whether the supplemental information reconciles to the financial statements or the underlying accounting and other records, as applicable, and performing procedures to test the completeness and accuracy of the information presented in the supplemental information. In forming our opinion on the supplemental information, we evaluated whether the supplemental information, including its form and content, is presented in conformity with the Department of Labor's Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974. In our opinion, the supplemental information is fairly stated, in all material respects, in relation to the financial statements as a whole.

/s/ GRANT THORNTON LLP

We have served as the Plan's auditor since 2020.

Chicago, Illinois

May 28, 2026

3

Packaging Corporation of America

Thrift Plan for Hourly Employees

Statements of Net Assets Available for Benefits

See accompanying Notes to Financial Statements.

4

Packaging Corporation of America

Thrift Plan for Hourly Employees

Statement of Changes in Net Assets Available for Benefits

See accompanying Notes to Financial Statements.

5

Packaging Corporation of America

Thrift Plan for Hourly Employees

Notes to Financial Statements

December 31, 2025 and 2024

6

Vesting

Participants are 100% vested immediately in the value of their contributions, earnings thereon and rollovers from other qualified plans.

The Company's matching contribution and the retirement savings contribution, and earnings thereon, vest at a rate specified in each Special Appendix. To the extent a participant is not 100% vested in the Company's matching contributions or retirement savings contribution, upon attainment of age 65 or termination of employment due to death or permanent disability, a participant will become 100% vested in the Company's matching contributions and retirement savings contributions. Forfeited non-vested accounts are applied to reduce future Company contributions.

Investment Options

Participants may elect to invest their account balances in any of the available investment options provided by the Plan through the PCA Defined Contribution Master Trust (the "Master Trust"). Participants may change their investment options on any business day, subject to certain short-term trading restrictions outlined in the Plan document.

The portion of the Plan's interest in the Master Trust currently invested in the PCA Common Stock Fund, and any future employee or employer contributions used to acquire PCA common stock, is invested in the Employee Stock Ownership Plan ("ESOP") component of the Plan. Plan participants may instruct the Plan's trustee to distribute future cash dividends paid on shares of PCA common stock credited to their PCA common stock ESOP directly to them. The election to receive cash dividends is made through the PCA Benefits Center, and dividends will be reported as taxable income.

Benefit Payments

In the event of retirement (as defined in the Plan), death, permanent disability, or termination of employment, the vested balance in the participant's account will generally be distributed to the participant or the participant's beneficiary in a single lump-sum cash payment. The portion of the participant's account invested in the PCA Common Stock Fund will be distributed in cash unless elected to be distributed in kind. In-service withdrawals of rollover contributions and related earnings and certain predecessor plan account balances, as defined, are available for any reason.

Certain participants, as specified in each covered location's Special Appendix, who have attained age 55 may elect an in-service withdrawal from their vested Company matching contribution account. Participants, as specified in each location's Special Appendix, who have attained age 59 1/2 may elect to withdraw all or part of their account balance. A participant's entire account balance shall be distributed no later than April 1 following the later of the calendar year in which the participant attains age 72 or the calendar year in which the participant's termination of employment occurs.

Administrative Expenses

Participant accounts are charged $12.50 per quarter for administrative expenses. If administrative expenses exceed the amount paid by participants, the Company will pay the difference. Administrative expenses primarily include recordkeeper fees, investment management expenses, and professional service fees.

Notes Receivable from Participants

Certain participants, as specified in each covered location's Special Appendix, may borrow an amount up to the lesser of $50,000 or 50% of their vested account balance. The minimum loan amount is $1,000. Such loans bear interest at the prime rate as published by The Wall Street Journal and are secured by the participant's account balance in the Plan. Loans must be repaid within 60 months, with principal and interest payments made primarily through payroll deductions. There may be primary residence loans that exceed the 60-month repayment period but only if they were transferred in from another plan. There may also be loans with repayment schedules affected by COVID-related relief that exceed the 60-month repayment period. Employees on an unpaid leave may continue to repay loans via check, money order, or online debit during their period of absence. Participants also have the ability to elect to make a one-time repayment of their outstanding loan balance, of which the payment can be made via check, money order, or online debit. Participants may take up to two general purpose loans at a time. A loan is considered in default and becomes a taxable event when a loan is not current at the end of the cure period, the quarter following the quarter in which the payment was missed.

Interest rates on loans outstanding in the Plan at both December 31, 2025 and 2024 ranged from 3.25% to 9.25%.

7

8

balances of the participants in the respective participating plan. The Master Trust specifically identifies contributions, benefit payments, and plan-specific expenses attributable to each participating plan. Investment gains (losses) are allocated to each participating plan in the Master Trust on a daily basis based on each plan's divided interest in the Master Trust. At December 31, 2025 and 2024, the Plan's interest in the net assets of the Master Trust was $1,458,721,397 and $1,262,435,669, respectively.

9

Fair Value Measurement

Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (i.e., an exit price). The fair value hierarchy prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets and liabilities (Level 1) and the lowest priority to unobservable inputs (Level 3). The three levels of the fair value hierarchy are described below:

Level 1 - Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities.

Level 2 - Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability. Level 2 inputs include the following:

Level 3 - Unobservable inputs for the asset or liability (i.e., supported by little or no market activity). Level 3 inputs include management's own assumption about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk).

Assets that are measured at fair value using the net asset value ("NAV") per share as a practical expedient are not categorized within the fair value hierarchy.

The level in the fair value hierarchy within which the fair value measurement is classified is determined based on the lowest level of input that is significant to the fair value measurement in its entirety.

The following is a description of the valuation techniques and inputs used for each major class of asset measured to estimate fair value by the Master Trust.

Self-directed brokerage account: Valued at the closing price reported on the active market on which the individual securities are traded.

Company common stock: Valued using the share value calculated from the observable market price of the PCA common stock.

Collective trust funds, target date collective funds, and short-term investment funds: Valued using the NAV per share practical expedient since these assets do not have readily determinable fair values and are subsequently excluded from the fair value hierarchy. In general, there are no restrictions on the redemption of these funds, which is permitted daily upon proper notice, and there are no unfunded commitments associated with these funds.

10

11

12

13

INDEX TO EXHIBITS

14

SIGNATURES

The Plan. Pursuant to the requirements of the Securities Exchange Act of 1934, the Benefits Administration Committee of Packaging Corporation of America has duly caused this annual report to be signed on its behalf by the undersigned hereunto duly authorized.

15