It's not often that I praise software. But Xournal++ has made my life so much easier recently.
I use it to fill in PDF forms. It allows you to enter text anywhere in the document. As well as allowing freehand drawing which is great for signing and ticking boxes.
I have recently had to fill in many forms and printing them out and scanning them back in was getting very tedious.
Any other program that I have tried to do this has not worked or is just really clunky. I even tried converting them to libreoffice draw/writer documents which is even more clunky! I have not tried any proprietary applications but can't imagine them being good considering the mess that is adobe reader.
Xournal++ is designed as a notetaking tool, it just so happens that it is also very good at filling in PDFs.