You can open the pdf file in an text editor like Notepad++, before saving it. A lot in a pdf file are just ASCII characters.
After retrieving, open also that version in the text editor and compare what has changed. That should give you an idea what went wrong.
I wrote a project on github, a C# sharp application, for viewing the actual pdf file content (i.e. the bytes it contains) and for extracting the text it contains (which is usually packed and can't be read with a text editor):
GitHub - PeterHuberSg/PdfParser