Adobe Acrobat can be used to secure PDF files. The full set of PDF security options is summarized here.

In most cases, you’ll likely want to encrypt a specific confidential PDF file with a password that is required open/view the file contents. To do so, follow these instructions:

1. Open the PDF in Acrobat DC.
2. Choose Tools > Protect
3. From Protect menu bar, select Encrypt > Encrypt with Password.
4. Check ‘Require a password to open the document‘ and type the password
5. In the Options section, select ‘Compatibility Acrobat X and later’ and ensure that ‘Encrypt all document contents’ is also selected.
6. Click [ OK ]. You will be asked to re-enter/confirm the password.
7. Click File > Save.
8. Confirm the password encryption was successful by closing and re-opening the document. Upon re-opening, you should be asked for the password.

Password Guidance:

Your password must be at least six characters long, but at least 10 is recommended. The password strength is displayed next to your password to indicate whether the chosen password is weak, medium, strong, or best. Save this password in a secure location and only share it securely with those you wish to have access to the document. (e.g. don’t put the password in the same email as the PDF attachment.  Send it via an alternate means of communication.)

If you are regularly sharing securely encrypted documents with a group, you can agree upon a common password to be re-used for each file.  It’s recommended that you rotate this password every 6-months

References:

Securing PDFs with passwords, Adobe Acrobat