Users are reporting an issue that after they received the (Adobe Sign) signed document via email and open it with a pdf reader, the document gets "locked", password protected and can't be opened again. We don't set password when sending the document for signing.
We managed to reproduce the issue, once the signed document is received via email and saved locally, and it's then opened with a pdf reader other than Adobe Reader the document gets password locked. If for example you want to rotate the document you receive a message that you can’t do that. But somehow the document is altered because next time you try to open the document you receive error message “pls enter the document password”.

See the attached screenshots for every steps we took:
1. Document signed with Adobe Sign, downloaded and saved locally. Open with a pdf reader, opens OK
2. Try to rotate the document, not possible. Click on Cancel and close the document – OK
3. Try to open the document again, you are asked for a password – NOT OK

Same password protection is being added if you preview the signed document from your email (so it's opened with a odf reader) and then you save it on your local drive. When you try to open it, it gives the same prompt "enter a password".

This is not happening when using Adobe Reader (rotating the page and then the document gets locked), but of course we need Adobe Sign signed documents to be able to open/rotated etc. with any pdf reader not just Adobe Sign. We can’t ask external users to use Adobe Reader only for the signed documents.