Comparison
When people search for an "open source PDF editor," they usually mean one of two different things: they want source code they can actually inspect, or they just want something free with no account and no strings attached. Those overlap a lot in practice, but they're not the same ask, so this page covers both.
PDFluent's desktop app is free for everyone, including commercial use, but it is proprietary software, not open source. If that's the deciding factor for you, be honest with yourself about it before you download it.
What is genuinely open source is the PDF engine underneath it. That same engine ships separately as a developer SDK under the AGPLv3, a real open source license, with the source available to read, modify, or embed in your own project if you're fine with AGPL's terms. The app you'd download to edit a file is proprietary-but-free; the code doing the actual PDF work is open, just packaged for developers rather than as a desktop editor.
Part of a genuinely open source office suite (MPL/LGPL). Can open and do basic edits on PDFs, but it's not built as a PDF tool specifically, so it shows its limits fast on anything beyond simple edits.
Open source (GPL), a small GUI for merging, splitting, and rotating pages. Does that one job well, mostly aimed at Linux users, not a general editor.
Open source command-line tools. Good if you're scripting something, not something you'd hand a non-technical person.
If you want a free, private, full-featured editor and the OSI checkbox isn't actually the requirement, PDFluent's app does that. If you specifically need to read or embed the PDF-processing code itself, the AGPL SDK is where that lives, separate from the desktop app. If neither fits and you need something with a fully open license end to end, LibreOffice Draw, PDF Arranger, or the command-line tools above are the honest options, each with the tradeoffs described.
Free for everyone, including commercial use. No account, no cloud.