Firefox can view PDFs so I assume Chrome also has that ability, and you're likely to already have one or the other if you're already using your computer for web browsing.

If you need something stand-alone, one of the Evince family might also suit your purpose. Evince = GNOME Document Viewer and XFCE is GTK-based, which seems like a good match. There's also Atril which is a MATE fork and Xreader, which is a fork of Atril, from Linux Mint's Xapps project.

LibreOffice, as mentioned by another respondent, has the ability to modify PDFs if not outright edit them, i.e. ability to place text boxes on top of the document, so you could theoretically use it to digitally complete a form you've been sent as a PDF and save it out as one. That may be worth jumping through the hoops if that sounds like a feature you might want.