This error message mean the PDF is corrupted. With each new version of Acrobat and Reader Adobe released over the last few years Adobe have tightened up the PDF Spec, i.e., badly formed PDFs that Acrobat and Reader used to be forgiving will no longer work.
Some clone PDF development products could be producing PDFs that don't conform to the spec.
It could also be that the PDF was corrupted during email transmission.
Find out if the PDF was developed with Acrobat or a clone.
It may also be that the file was sent by e-mail and either not properly encoded at the sending end or your e-mail client decoded it incorrectly. It used to be common for clients to try to send PDFs as ascii files and that was the source of the corruption - a PDF is binary.