Are you viewing the PDF file in the Dropbox application?
If so, the problem is likely that the Dropbox application's viewer is ignoring annotations. Open from Dropbox in Adobe Acrobat Reader and the annotations should appear.
Thank you for the very quick answer.
No, in my case the problem was likely that it was tacking some time for the upload (I have an ADSL connection, that is very slow in uploading) and my phone was killing the apps in the background (some android vendors do that to save bactery).
I have to either keep the app on after the edit or pou it in a list of "protected apps" that android doesn't kill.
So, I can confirm that when the document is edited in acrobat reader in android, than the document is uploaded to dropbox automatically.
As an academic, I have heavily used Acrobat Reader on my Lenovo Tablet (Android 6.0.1) to mark up PDFs using the 'Comments' feature. Now, the new version won't allow me to save the comments (writing, sketches, highlights) and I lose my work every time I try to save it by going to the upper left-hand corder and hitting 'my documents' (in the past, that was how it would save and sync with DropBox). I just keep losing the marked-up documents and wasting my time. Please help!