@creamqueen
For just filling a form that has clickable fields, I use Xreader (included by default in Linux Mint). Okular (Windows/Linux) or Adobe Reader (Windows/macOS) also work for this, depending on your OS.
For adding a signature without having to print and scan again, I use Xournal. It's also good for adding text manually when the PDF doesn't have fillable fields.
If the above don't do what you need, you could try messing around with LibreOffice Draw (cross-platform).