Linux

Most more advanced pdf handling tools I've used on linux were cli based.

Not ideal.

Were I responsible, I think I might look into creating a self-contained executable, wine wrapper and all, for whatever windows editor ends up being used. That way IT can forget about setting up wine on each machine, and just ship the whole thing ready to run. For updates, just rebuild the executable with the new exe or wine version.

PDF is such a mess of a format, feature complete "editors" are few even on windows, and essentially a giant collection of hacks around the limitations and features of the format. I'm not aware of anything linux native that's even close to parity.

Excellent question. You may find the answers disappointing. Personally I use xournal for pasting signatures (sigh) into PDFs. UX is rubbish but it works. I do a bunch of other stuff to PDFs with command-line tools, which is extremely efficient and fast once it's set up. Doubtful this will fit your requirements, alas.

I have used Master PDF Editor happily for several years.

I don't work in the legal field, and I can't/don't affirm that it meets all of your requirements.

okular supports signing. But you have to set up your signing certificate outside it and then point it at the certificate database in the backend settings. Creating a signing certificate in adobe, saving that to a file, registering that file with nsdb, setting nsdb as backend in okular, works. Only have to do the setup obcr, then just works.

Doesn't master pdf editor do all that?

You work for a legal firm?

While I fully support the embracing of Linux by the corporate world, make sure clients and especially courts can actually use the files your firm sends out.

if a combination of 2 programmes work, then pdf arranger and libreoffice draw / xournalpp / okular. both foss and free (As in beer). for stuff like rearranging pages, changing sizes or orientation, cropping or even signing, the former can do fairly easily. for highlight, redact, bookmark, signing, latter work (they are a slighlty powerful viwers essentially, whereas libreoffice draw can edit much like adobe assuming fonts are availables).

it would make the workflow harder, but this is roughly what i use. I do not have to do pdf manipulation much, mostly read, so i have a simpler viewer (zathura in my case), which you can replace with lets say okular and get most of the latter set of features. and whenever you have to edit, you can make some shortcut (keyboard) to open current file in pdf arranger and there do former set of operations.

if you are okay with webapp, then i think xodo should work (they have a desktop electron client as well, but i think they only give windows download options). there you get all features, but would likely have to buy licenses. do not know where they are from, but i think they maybe are from china.

As for distro, and if you are tech support, see if you could go with a immutable distro, as that may reduce your work. (i do not want to start a distro war here so not giving any recommendation).