In general, we follow the Chromium Contributing guidelines in PDFium. The code review process, and the build tools are all very similar to Chromium. The PDFium README outlines specific build and test information for PDFium.

This document focuses on how the PDFium project operates and how we’d like it to operate in the future. This is a living document, please file bugs if you think there are changes/updates which could be put in place to make it easier to contribute to PDFium.


When writing a new feature or fixing an existing bug, get a second opinion before investing effort in coding. Coordinating up front makes it much easier to avoid frustration later on.

If it‘s a new feature, or updating existing code, first propose it to the mailing list.

  • If a change needs further context outside the CL, it should be tracked in the bug system. Bugs are the right place for long histories, discussion and debate, attaching screenshots, and linking to other associated bugs. Bugs are unnecessary for changes isolated enough to not need any of these.
  • If the work being implemented is especially complex or large a design document may be warranted. The document should be linked to the filled bug and be set to publicly viewable.
  • If there isn't a bug and there should be one, please file a new bug.
  • Just because there is a bug in the bug system doesn't necessarily mean that a patch will be accepted.

Public APIs

The public API of PDFium has grown over time. There are multiple mechanisms in place to support this growth from the stability requirements to the versioning fields. Along with those there are several other factors to be considered when adding public APIs.

  • Consistency. We try to keep the APIs consistent with each other, this includes things like naming, parameter ordering and how parameters are handled.
  • Generality. PDFium is used in several places outside the browser. This could be server side, or in user applications. APIs should be designed to work in the general case, or such that they can be expanded to the general case if possible.
  • Documentation. All public APIs should be documented to include information on ownership of passed parameters, valid values being provided, error conditions and return values.
  • Differentiate error conditions. If at all possible, it should be possible to tell the difference between a valid failure and an error.
  • Avoid global state. APIs should receive the objects to be operated on instead of assuming they exist in a global context.


There are a lot of consumers of PDFium outside of Chromium. These include LibreOffice, Android and offline conversion tooling. As such, a lot of care is taken around the code in the public folder. When planning on changing the public API, the change should be preceded by a bug being created and an email to the mailing list to gather feedback from other PDFium embedders.

The only stability guarantees that PDFium provides are around the APIs in the public folder. Any other interface in the system can be changed without notice. If there are features needed which are not exposed through the public headers you'll need to file a bug to get it added to the public APIs.


All APIs start as Experimental. The experimental status is a documentation tag which is added to the API, the first line of the API documentation should be // Experimental API.

Experimental APIs may be changed or removed entirely without formal notice to the community.


APIs eventually graduate to stable. This is done by removing the // Experimental API. marker in the documentation. We endeavor to not change stable APIs without notice to the community.

NOTE, the process of migrating from experimental to stable isn’t well defined at this point. We have experimental APIs which have been that way for multiple years. We should work to better define how this transition happens.


If the API is retired, it is marked as deprecated and will eventually be removed. API deprecation should, ideally, come with a better replacement API and have a 6-12 months deprecation period. The pending removal should be recorded in the documentation comment for the API and should also be recorded in the README with the target removal timeframe. All deprecations should have an associated bug attached to them.


In order to allow the public API to expand there are version fields in some structures. When the versioned structures are expanded those version fields need to be incremented to cover the new additions. The code then needs to guard against the structure being received having the required version number in order to validate the new additions are available.

Trybot Access

Changes must pass the try bots before they are merged into the repo. For your first few CLs the try bots will need to be triggered by a committer. After you've submitted 2-3 CLs you can request try bot access by emailing one of the OWNERS and requesting try bot access. This will allow you to trigger the bots on your own changes without needing a committer.


All changes committed to PDFium must be reviewed by a committer. Committers have done significant work in the PDFium code base and have a good overall understanding of the system.

Contributors can become committers as they exhibit a strong understanding of the code base. There is a general requirement for ~10 non-trivial CLs to be written by the contributor before being considered for committership. The contributor is then nominated by an existing committer and if the nomination is accepted by two other committers they receive committer status.


The OWNERS files list long time committers to the project and have a broad understanding of the code base and how the various pieces interact. In the event of a code review stalling with a committer, the OWNERS are the first line of escalation. The OWNERS files inherit up the tree, so an OWNER in a top-level folder has OWNERS in the folders subdirectories.

There are a limited number of OWNERS files in PDFium at this time due to the inherent interconnectedness of the code. We are hoping to expand the number of OWNERS files to make them more targeted as the code quality progresses.

Committers can be added to OWNERS files when they exhibit a strong understanding of the PDFium code base. This typically involves a combination of significant CLs, code review on other contributor CLs, and working with the other OWNERs to work through design and development considerations for the code. An OWNER must be committed to upholding the principles for the long term health of the project, take on a responsibility for reviewing future work, and mentor new contributors. Once you are a committer, you should feel free to reach out to the OWNERS who have reviewed your patches to ask what else they’d like to see from you to be comfortable nominating you as an OWNER. Once nominated, OWNERS are added or removed by rough consensus of the existing OWNERS.


There are times when reviews stall due to differences between reviewers, developers and OWNERS. If this happens, please escalate the situation to one of the people in the top-level OWNERS file (or another of the owners if already discussing with a top-level owner). If the disagreement has moved up through all the OWNERS files in the PDFium repo, the escalation should then proceed to the Chromium ATL_OWNERS as the final deciders.

The Standard of Code Review document has some good guidance on resolving conflicts during code review.


All contributors must complete the Google contributor license agreement. For individual contributors, please complete the Individual Contributor License Agreement online. Corporate contributors must fill out the Corporate Contributor License Agreement and send it to us as described on that page.

Your first CL should add yourself to the AUTHORS file (unless you’re covered by one of the blanket entries).

External contributor checklist for reviewers

Before LGTMing a change, ensure that the contribution can be accepted:

  • Definition: The “author” is the email address that owns the code review request on
  • Ensure the author is already listed in AUTHORS. In some cases, the author's company might have a wildcard rule (e.g. *
  • If the author or their company is not listed, the CL should include a new AUTHORS entry.
    • Ensure the new entry is reviewed by a reviewer who works for Google.
    • Contributor License Agreement can be verified by Googlers at http://go/cla
    • If there is a corporate CLA for the author‘s company, it must list the person explicitly (or the list of authorized contributors must say something like “All employees”). If the author is not on their company’s roster, do not accept the change.

Legacy Code

The PDFium code base has been around in one form or another for a long time. As such, there is a lot of legacy hidden in the existing code. There are surprising interactions and untested corners of the code. We are actively working on increasing code coverage on the existing code, and especially welcome additions which move the coverage upwards. All new code should come with tests (either unit tests or integration tests depending on the feature).

As part of this legacy nature, there is a good chance the code you’re working with wasn’t designed to do what you need it to do. There are often refactorings and bug fixes that end up happening along with feature development. Those fixes/refactorings should be pulled out to their own changes with the appropriate tests. This will make reviews a lot easier as, currently, it can be hard to tell if there are far reaching effects of a given change.

There is a lot of existing technical debt that is being paid down in PDFium, anything we can do here to make future development easier is a great benefit to the project. This debt means means code reviews can take a bit longer if research is needed to determine how a feature change will interact with the rest of the system.