I spent 2 years testing free annotators across Windows, Mac, iPad, and Android before realizing that no single tool covers every use case. On my desktop, I use Xournal++ for drawing freehand diagrams during technical reviews, while on iPad, PDF Expert handles highlighting and sticky notes well, though the free version caps you at 10 annotations per document - enough for light editing but not heavy collaboration. For Android, Xodo syncs smoothly with Google Drive, which saves time when I switch devices mid-project. When I need to export annotated files for engineering teams, I use WordPDF to convert my marked-up pdf to dxf for vector extraction, and the 48-hour auto-delete keeps my draft notes secure. The real trick is matching tools to tasks: readers for quick review, annotators for feedback, and converters for final output.