I am actually only commenting about regular behavior for which normal, regular people have very little options. I believe my example about buying a house is pretty clear how personal data is shared through channels with very little to no supervision. Just like pdfescape.
Also, I was pretty clear about simply using the PDF reader of Firefox which now allows form filling. No need for an online service. I would not use credentials or stuff that people can actually use to make a transaction on pdfescape.
My goal is merely to put things into perspective. Of course you should always use local solutions. For that reason alone I run my own server, have my own selfhosted filecloud, selfhosted password manager and local-only MFA app (Aegis). So yes, I do everything I can as long as it is practical enough to stay secure.
But my example about the intermediary is pretty clear. At some point, even though I host everything locally, even I am forced by law to use entities that I have no clue about what they will do with my very sensitive data and they are not actively regulated by data protection authorities or EBA regulations. With that in mind, I have no problem using pdfescape.com to fill in just my name and IBAN, since no one can use it to harm me or steal my identity (they need a bit more for that).
/offtopic
I do find it saddening there still is no proper PDF editor for Linux.
I get that, but without providing actual, useable alternatives (for Linux) people will have to keep using services like pdfescape and will have to take these risks. Or buy a Mac or switch back to Windows.
KDE user here, I use Okular for PDF reading and annotating and for PDF writing/editing I use Inkscape. So far it has done the job for me. The only inconvenience is that AFAIK it allows only one page as input and only one page as output, so after that I usually stitch back together the pages with a one-liner in terminal. Luckily forms aren’t more than 5-6 pages, so it’s not that time-consuming.
There are a number of file viewing programs available for Manjaro that will do what you need. Okular, Atril, Evince, Xreader all fill in pdf forms. You just need to choose which one you like best. Four possible solutions from a simple Google search. Be sure to look at the dependencies for each one before installing.
All of these applications do not allow you to save tables in document forms after filling them out . I have tried. Only Adobe Acrobat allows you to do this. On Android, Adobe Acrobat works