what elcreative said.
I have also tried some scanned-as-images-pdfs and yes of course the fonts can not be enlarged.
A properly created pdfs can be enlarged, but depending on the editing it will not reflow correctly, e.g wrong page/paragraph breaks, overlapping fonts etc.
At least I know one of my pdf breaks like that.
OCRed pdfs, if not edited properly will result on this case.
This is only my opinion though, I don't see converting them to epub will help. But if you are willing to put time to it you can edit the epubs afterwards, see
Sigil on that.
pdf editors exist but they are very pricey.