Some PDF files are damaged. Adobe Acrobat for Linux says:
"There was an error loading this document. Could not
repair file."
And xpdf says:
"Error (0): PDF file is damaged - attempting to
reconstruct xref table...
Error: Couldn't find trailer dictionary
Error: Couldn't read xref table"
Those are unreadable in the latest (20 May 2001) release:
apachenote.pdf
editor.pdf
xml-howto.pdf