When I preview my email attached PDF file I can read it properly. However, when I download this PDF file into my PC I cannot open the file at all.

When I preview my email attached PDF file I can read it
properly. However, when I download this PDF file into my PC I have the
following error and cannot open the file at all: "There was an error
opening this document. This file is damaged and could not be repaired". I am
using Adobe Acrobat Reader DC product. May I get help on this?