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One open source software to add annotations under Linux is okular (https://okular.kde.org/).
One can add annotations easily (go to Tools, tick review, or just type F6).
Then, it is time to save your document or to send it to a collaborator but wait a minute: we do not see the annotations when using acroread reader !! No worries, plenty of resources tell you to go to File/save as
Seems to work indeed. You quit, open the file and there you can see the annotations. Now, I use xpdf to read the PDF file. And here nothing. Oh, and I send the PDF to a journal review; they include the PDF inside another one and there no annotations either….
Final solution: instead of File/Save as, just print the document in … a PDF file: File/Print
This worked for me.