Different PDF viewers repair (or ignore) different kinds of damage in the PDFs they show. So if you try viewing your document in another viewer, it might open it and you might see something, maybe even the expected content.
I would not trust the document contents, though. Manipulation attempts, in particular post-signature ones, sometimes make use of repair mechanisms in PDF viewers to show something different than there is in the original document.
Thus, you should ask the source of that PDF to again send you a PDF but to make sure that it doesn't get damaged (maybe using a different signature software, maybe sending the document to you via a different channel, ...).