If you received an email about a shared document or sign request in DocHub and are unable to access the PDF to sign, view, or edit after clicking the blue View Document button in the email, then the file has most likely been deleted and removed from DocHub, and you should contact the owner of the document by replying to their shared document or sign request email. The email is sent from DocHub, but the reply-to address is set to their email address. If the document URL is displaying a 404 error, that it most likely the case, but you can also contact our support team just in case to report a possible outage or issue with that user's account or document.
If you can't access a PDF in DocHub after you were given a DocHub document URL and never actually received an email from DocHub and if you only see a "You Shall Not Pass" message at the URL, you must not have permission to view or edit the PDF. You can view our article about How do I share or download my edited PDF? for more information about how sharing in DocHub works and the privacy measures we use which involves adding a document aka authentication token to the URL for each shared document and sign request where it's a long string of letters and numbers at the end of URL that bypasses the need to be logged in to DocHub. You would need a URL with this long string at the end for any document that is set to Private.