Encrypting PDF documents in Preview
To encrypt a PDF document that contains a form that you filled out with the Preview application:
1. Choose "Print..." from the File menu.
2. In the sheet that appears, choose "Save as PDF..." from the PDF button in the lower left corner.
3. In the Save panel that appears, click the "Security Options..." button in the lower middle of the panel.
4. Complete the panel that appears according to the security restrictions you want.
5. Save the PDF from the Save panel in step 3.
When you open the document back up, you will be prompted for passwords according to the restrictions you specified. The document's entered text should be just as you saved it.
*Warning: If you choose to password protect the ability to print the document, and re-open the encrypted document and try to print it, it asks you for the password. Even if you enter the password, the document won't print. The encryption permissions in the Inspector floating panel for the document will also say that you don't have the ability to print. Seems like another bug.*
I would think that encrypting a document should also give you the ability to password protect it. Having to act like you're printing the document seems unintuitive. I hope these bugs get fixed soon.
No sense posting here unless you have a problem. If you want to report this to Apple, either send Feedback or send bug reports and enhancement requests via its Bug Reporter system. To do the latter, join the Apple Developer Connection (ADC)—it's free and available for all Mac users and gets you a look at some development software. Since you already have an Apple username/ID, use that. Once a member, go to Apple BugReporter and file your bug report/enhancement request. The nice thing with this procedure over submitting feedback is that you get a response and a follow-up number; thus, starting a dialog with engineering.