I cannot emulate the fault. Windows mail is not my default, but useable. Perhaps Celestra's suggestion to update your Adobe is the answer. But what program are you using to save the scanned doc as a PDF?
A basic, but maybe not helpful, suggestion, would be to download and install Windows mail live. It is a better program all round (and free!)
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But if you have a PDF problem, rather than a mail attachment problem, it wont help.