When I add some annotation to a pdf, this pdf will be automatically convert to an uneditable file

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When I add an arbitrarily annotation to this pdf, the pdf will be automatically convert to an uneditable file. For other files, everything is fine.

By the way, the original file is less than 1MB, but the converted file is 10.1MB.

This has been previously discussed; see this post for a summary, and perhaps take a look at the whole thread. A quick search for “annotation file size” reveals that and other relevant threads too.

Which version of macOS do you use and what kind of annotation? I just tried this on macOS 11.4 using various annotations but the PDF document remains editable after saving, closing & reopening.

I use macOS Big Sur, just highlight.

This is the original file I’ve downloaded. It is editable.

When I highlight the title, and the file size become almost 30 fold than before and uneditable.

The file size is increased, quite likely due to a difference in compression algorithms used and PDFKit, but I’m not seeing an issue highligthing that PDF.

There is no issue when I try to annotate a file, but when I save it, it will become uneditable.

There is some issue with the PDF itself.
Download a new copy, open it in Preview, and try to select text on the second page. Are you able to?

No, I already try this way. It not works.

Correct. So there’s something specifically wrong with the PDF, not DEVONthink itself. Or it’s potentially incompatible with Apple’s PDFKit framework.