Whenever you are trying to open a PDF file on any website link an error is being displayed while opening a PDF file. Following is example of the error displayed during attaching PDF file on the Website.

“File does not begin with '%PDF-
The document '[filename]' could not be opened, because the application program that created it could not be found
Error in zip use PKZipFix
Cannot open file; it does not appear to be a valid archive
System Error -36”

One way to solve this issue is try to confirm that you are using the right application to open the file and reinstall the application you are using to open the file. And try to send back the file again with the help of encoding software which will format your e-mail application and different encoding tools are like UUEncode, BinHex or MIME. And confirm whether your e-mail has enough storage capacity to accept the attachments or else check whether the sender must have resend the file due to damaged and confirm whether it has .pdf file extension and not the .mme file extensions.