[BRX]

Sciurius posted an SQL statement there that does such a selection.
That works fine so far, but it's probably not for the average user.

Indeed. I'm looking for things to do from within the app.

itsme

"missing PDF" is imho covered by "Find Missing Files"

Well, "find missing files" is the reason I came up with the idea because I want to have a way to conveniently save the results (in a collection) and to fix manually later. On W10 (haven't tried Android) you get just a list of links with missing files with the option to delete them. Well, I'd like to fix them manually with providing the file or swap file. At least a way to add the results into a collection is needed IMHO.
And the other things like notes, crops etc. of course would be possible with filters as well. That's why I talked about filters and/or "system collections".