2. In the "Document Properties" interface, click the "Security" option to get the "Document Permission Details" of the current pdf document.
3. Click "Security Method" and select "Security Password" or "Security Certificate" to achieve stronger encryption protection.
4. In the "Password Security Settings" interface that pops up, select "Restrict modification and printing of documents, and password is required to modify permission settings", and set the corresponding modification permission password. Click the Save button to exit.
Besides encryption, you can also add a "digital signature" to the PDF document, thus enhancing the copyright protection of the PDF document.
The above are the specific steps to encrypt and save PDF files. We can try to follow the above steps.