Solution
Download the document first, and then open the document or use a browser that has a compliant PDF viewer.
Generally, you get that message (About a password ) when you attempt to open a pdf file in a browser that uses a non-compliant pdf viewer and generally, the pdf file is a fillable form of some type. In order to solve this please download the PDF first and then open it
https://community.adobe.com/t5/acrobat-reader/cant-open-a-pdf-file-in-gmail/td-p/9235779?page=1
It is also suggested that you edit your email form to alert customers of what to do if they come across the password request. This will limit the number of calls your company will receive on the issue.
