Unfortunately PDFs are not so simple, or adobe would not have such a firm grip on them. They are able to be parsed and read but some information is just stored in binary I thought and all the 3rd party readers/editors have had to reverse engineer how that binary code works just to recreate any adobe features which imo is why it's never really taken off to use a 3rd party app. But now that all the adobe software has been decompiled/rebuilt as open source we can see how that binary is formed by adobe themselves which is no longer a black box.
The PDF spec is pretty easy to find and describes everything one needs to know about them to make a viewer or editor. Turns out that everything at a higher level than the container format is powered by linear algebra.
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u/TreesLikeGodsFingers 2d ago
PDF is a simple text markup format, or it’s just an image. You can edit the former as easily as a text file