You have probably already tried this, but just in case you missed it: Above the pdf itself, there is a toolbar. The part of the bar directly above the pdf shows a box with "70%", which is the zoom ratio. If you scroll all the way to the right, there are some editing tools on this bar. If you click on the tool that looks like an I-beam, you will get a cursor that you can place where you want to enter text, and then you can type. If you click on the tool that looks like a pencil, you will be able to draw with the mouse. (I have not found a tool that lets you neatly check a checkbox.)

I hope this helps.

See also "Annotate PDF files":

• https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-or-choose-another-viewer