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Rather common forum recurrence I am familiar with,
It has to do with config/association settings on your computer & browser. (.pdf vs .pdf.xxx). Mentioned earlier.
Not something paid a whole lot of attention to nowadays.
As long as it is a valid(non corrupted) file ending with just a .pdf and your browser is auto-associated with a PDF viewer, it will open up.
Otherwise it does not know on what application to use thereby prompting the save dialog.
Those w/out a problem opening the file, indicate that at some browsing/opening point they must have "always- associate" it with a PDF viewer.