Screen shot of PDF Error on Uniforms of the United States Marine Corps article on enwiki
Here's the error message word for word:
POST request failed
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The POST request to http://pdf1.wikimedia.org:8080/mw-serve/ failed (Operation timed out after 3000 milliseconds with 0 bytes received).
Return to Main Page.
Apparently, I'm not alone: There were postings to Wikipedia Help Desk [http://en.wikipedia.org/wiki/Wikipedia:Hd#PDF_won.27t_appear] and Village Pump (technical) [http://en.wikipedia.org/wiki/Wikipedia:VPT#.22PDF_version.22_option_not_working]
It also seems that other wikis are having the same issue.