Our favorite is still PDF-XChange [1] which has been our daily driver for years. Only dislike is the difficulty in opening a document in a separate application window. It's either everything in one window or everything in its own window.
Ohh, daily driver here. You can configure external apps to open the pdf (in a given page) and choose your hotkey for it, and with some effort, that supercharges the reading. With some scripting and using the grand pymupdf library one can customize it as deeply as one wishes.
ok thank you so much... finally an open source pdf reader thats not acrobat that works (hopefully, still testing). on the issue of pdf forms, my gosh they just should not exist, thats what web pages are for.
pdf forms are pretty convenient when you need to fill it in, print and send to government by snail main or show in person or sent digitally. Happens in some places more often than in others.
They're also amazing for digital TTRPG sheets. You can send around the PDF form, and everyone can fill out and store their own sheet, print it, send it back to the GM, whatever the group needs.
To be clear, there are multiple kinds of PDF form. GGP might have had experience with one of those that are actually supposed to be submitted over the Internet straight from the PDF reader. (Not only is this a thing that Adobe did to their format, it is IIRC a thing that they did to it in more than one way.)
Adding content to a PDF, such as a signature, falls under the PDF editor umbrella. Sumatra PDF is purely a PDF reader, so (sadly) does not implement any editing features.
[1] https://www.pdf-xchange.com/