It sounds like the PDF file is not really a form, just a digital sheet(s) of paper. Copying to the clipboard won’t really help if that’s the case.
What you need is the Typewriter function, and the only free PDF viewer I know that has it was recommended to me from this board. Try PDF X-change Viewer. Use the typewriter function, and don’t be afraid to mess with the font or size to make it fit. When you save the file, it will include what you’ve written on top of it.
BTW, I do not recommend making it your default PDF viewer, as it does take longer to load. Just use it when you need to write on top of a PDF.