Next, select the location where you want to save the file. Remember to set the save format as "PDF" . Then select the "Options" button, which is also located in the file save dialog.
The "Options" dialog box will appear, locate and tick the "Encrypt the document with a password" line. Then click OK.
The password settings dialog box will appear. Please enter the password you need to set up in the PDF file and click OK to save.
Back in the Save as dialog box, click "Save" to save the file.
That's it, now try opening the file earlier.
The dialog box asking for a password will appear. Pretty simple, isn't it?