While it is true that PDFs can be edited, the beauty of the format is that it is encoded in a way that it looks the same in any computer. Unlike word, where for example, if you don't have the font which the author intended, the document can look totally different. So the appeal is not in whether or not it can be edited, but that it will retain it's format.
Secondly, it's not like jpgs or other scanned documents cannot be edited, if someone is good enough with computers to edit pdfs, they are probably IT aware enough to edit scanned documents too.
In the end, it just comes down to whether or not someone intends to break trust (or the law, where applicable) by editing documents. PDFs were never intended to be uneditable. One can easily create fake documents, whether or not they're in soft copy or hard copy. Its the intent that counts.
As for the person who commented about the pdf not uploading - there could be too many possibilities of why it was not accepted by the system. As someone else mentioned, try other ways by the re-encoding the pdf (i.e re scan the document, or compress it, or Google the various options available. Sometimes, pdfs encode an 'unacceptable' string, or there was corruption during the encoding, or something else, you just can't say.
