I had a Mime-type problem when downloading pdf from Sitecore site the following error showed up in the log:
Sitecore MimeConfig
This turned out to be a problem with MIME-types, and to solve it download the MimeConfig and add a reference to it in web.config as described in this Sitecore documentation.
Open the file and make sure that a mime entry for PDF exists and make sure it is not commented out.
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