PDF is only open in the sense that .docx and other MS Office formats are open - there is a spec, and you are allowed to implement it, but it is almost impossible to implement 100% of the features. Programs like Evince (Poppler) and Sumatra have come a long way. They are really excellent and read 95% of PDFs, many even faster and with less RAM usage than Adobe, and almost 100% of static PDFs ("print to PDF").

But for the occasional PDF form, if I don't want to print it and fill it out by hand, I have to use Adobe (Poppler sometimes gets the font size wrong so you don't see when you are overflowing a field, code that checks whether you filled it out correctly doesn't work, and highlighting that tells you which fields are for what kind of user to fill out is also often messed up).

Actually what I just do is to run Adobe Acrobat Pro in wine, which allows me to actually save arbitrary filled out forms.

Finally, one ironic benefit of the Adobe Reader is that it is distributed in binary form. If you are stuck on a Linux distribution a few versions back, you can just download and extract it to your home directory and run it from there. I realize that from-source or package manager purists might cringe at that thought, but it actually saved me a couple of times when I just had to print out a stubborn PDF (and couldn't ask the admin to kindly update my distribution at work...).