I'm not sure about BP's built-in functions with respect to PDFs. As for the 3rd party DLLs it is my understanding that the purpose of being allowed to import dlls in object studio is to add 3rd party extensions. I imagine it's the same logic that went with us being provided code stages, that Blue Prism knew they couldn't think of every possible situation that would need automated, so code stages and dll imports give us the ability to add functionality ourselves.
If I understand it correctly, we can ues dlls from whatever source, even make them ourselves, but we can't expect Blue Prism to support it of course. So using a dll will probably be fine, but if something breaks or there are issues I imagine you'll be on your own.