Open Source PDF Editor
The PDF is everywhere precisely because it's meant to be final, which makes editing one feel adversarial - and the common workaround, uploading a contract or scanned ID to a free web tool to merge or sign it, hands the most sensitive document you have to a stranger's server. The open source editors here annotate, merge, and sign PDFs entirely on your own machine, so a signature or a confidential page never makes a round trip through someone else's cloud.
Stirling PDF
Self-hosted PDF platform with 50+ tools, a browser UI, and a private API
OCRmyPDF
Command-line OCR for scanned PDFs that adds a searchable text layer and keeps the original images intact
Xournal++
Handwriting note-taking and PDF annotation app with pressure-sensitive pen support
Sioyek
Keyboard-driven PDF viewer built for reading textbooks and research papers
ONLYOFFICE Docs
Collaborative online office suite for documents, spreadsheets, presentations, forms, PDFs, and diagrams
PDF Arranger
Small GTK app to merge, split, rotate, crop, and rearrange PDF pages
PDFsam
Desktop PDF tool for splitting, merging, mixing, rotating, and extracting pages
Paged.js
JavaScript library for paginated browser layouts and print book PDFs using HTML and CSS
Okular
Document viewer that opens and annotates PDF, PostScript, comics, EPUB, Markdown, and images