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The original was posted on /r/opensource by /u/Anxious_Reporter on 2023-08-01 01:33:21.

I currently use the Xodo free desktop app for annotating PDFs and I think it’s pretty good, but I find their recent change in the online version of the app (requiring you to pay for performing more than “1 action a day” unless you pay their $100/yr subscription fee) extremely annoying. I’m curious why no other PDF annotation software (open source or otherwise) seems to have as many features.

I find that there is not really anything similar to it (when just Googling something basic like “best free PDF annotation software”) in terms of being able to:

  • Add transparent text box annotations --with varying options for sizes and colors
  • Add freehand annotations / drawings --with varying options for line thickness and colors
  • Add clickable hyperlinks
  • Highlight or underline (w/ optional squiggly line) existing text --with varying options for colors
  • Click and drag over existing text to add highlights or underline annotation to
  • Strike-out / cross-out existing text
  • Add shapes like boxes, circles, lines, and arrows- --with varying options for colors
  • Add images on top of PDFs like transparent or non-transparent JPEGs and PNGs
  • Erase annotations
  • Multi-select annotations
  • Move/drag selected annotations around
  • Add Bookmarks / Outlines from existing PDF pages (PDF terminology seems to conflate these two words, but I think I more mean “Outlines”) --eg. so the PDF has a viewable and clickable table of contents view
  • Save (and in the case of a web app, download) all of these changes onto the existing PDF itself, with the annotations still editable if I open the PDF again in the app even on another device or in the webapp.

I’m just curious, is this a technically challenging thing to do on PDFs? I don’t have much experience working with the underlying mechanics of PDFs, but it seems like this shouldn’t be that hard, yet these features seem uncommon (at least to find them all in a single application).

Many alternatives seems to not be able to do most of these things (eg. Okular creates external local data that must be opened along with the edited PDF in order to see the changes and only works when using that specific app unless you “print” the PDF which flattens all annotations and makes them permanent and also removes bookmarks).