On Linux it is already kinda hard to edit PDFs.
- GIMP for blackening and compressing with jpg
- PDF arranger for concatenating PDFs, resizing pages, deleting pages, adding images as PDF pages
- Firefox PDF editor for drawing and text
- maybe OnlyOffice for all of it?
- MasterPDF is probably a fine proprietary tool
But on Android, there is nearly nothing
StirlingPDF is a great tool you can selfhost with docker or podman. But the "draw and write" editing seems to only work on desktop browsers".
There are many viewers but all editors are not only proprietary, but force you to login or pay too.
Of all of them, I found XODO PDF Editor to be the least bad (Foxit doesnt launch without loading code from RAM, Adobe will have other issues).
And of course, don't enable internet access for the app. This filters out the bad ones immediately. A PDF editor doesnt need internet access.
It has a ton of features like adding texts and annotations, signatures etc.
If you can convert a PDF into an image, you can use the various image editors (simple gallery, image toolbox, new gallery, signal) to censor parts and then convert back to PDF.
Always be aware that censoring only wors if you flatten the PDF! PDF is just a container and can contain SVGs which can have layers.
If you want to censor digital text, you need to convert it to raster / image first!