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App to fill in and sign PDFs
(jlai.lu)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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The thing that you're looking for is called "PDF forms".
I can't find an app by this name. What repository is it in?
It's not an app, the technology you described is called a "PDF form".
So it's not the thing they were looking for
It appears that the OP is incapable of discovering Adobe Acrobat.