t's a strange feature but Gmail app downloads only certain files (such as .jpg, .png, etc)
There are several ways to solve it. In addition to apps mentioned in previous post in this thread you may use the following...
Install Astro app and in it's settings/preferences click on Enable Browser Download. Astro will inform you that this is conflicting with preview, but dont't worry.
After this operation you'll still have just a Preview button when you open the mail with attachement in your Gmail app, but if you click on it you will have a popup asking you if you want to open the file with the associated application (Adobe Reader for .pdf for instance) or you will use Astro. If you choose Astro your file will be downloaded to download folder, where you can handle it further....
Hope it's simple
Bye