Encrypt

This question has nothing to do with the Acrobat Reader mobile app.

You should consult with Apple support.

See here:

•  https://support.apple.com/guide/pages-ipad/password-protect-a-document-tanca246d3ac/ipados

Now, if you're actually asking about how to protect a PDF with a password using the Acrobat Reader mobile app ( so that no one can open it),  that is a new feature that was introduced in the last update. 

When it comes to using the term "encrypt' interchangeably with "password protected", it creates confusion;  since when you apply  password protection to a document file it  doesn't necessarily mean that it is also applying encryption to a document. 

If you're using the Acrobat Reader mobile app in an iOS device, open a PDF file and see if you get the following options (see slides below; at least in Android this is a new feature that was introduced in the last Acrobat Reader mobile app update):

Not possible with Acrobat Reader.