When a PDF is oneboxed, the URL is used to determine the onebox contents (see Oneboxing of PDFs and other attachments). The URL seems to be used as basis of the contents.
However, characters that are special for a URL do not seem to be correctly translated.
For example, the URL https://documentation.invantive.com/2017R2/exact-online/Exact%20Online%20API%20Data%20Model.pdf leads to box with “%20” in it:
I propose to rewrite the escaped characters back to their original character, such as “Exact Online API Data Model.pdf” in this case.