After updating to any of the 2.7.x versions when downloading a PDF file i get the same result: There was an error opening this document. The file is damaged and could not be repaired.
I migrate all files to new version from Admin Menu Downloads Settings Migrate Tab
I saw in another thread that it was a WP Super Cache related issue and to activate: Don’t cache pages with GET parameters. (?x=y at the end of a url)
I did and it’s not working, I even desable WP Super Cache plugin and get the same result
http://www.salespartnersmexico.com/descargas/