I don’t know about High Sierra but with Sierra, the latest encrypted PDFs just show up as greyed documents in the MacOS Preview and even if you file open in Safari. They do work in Adobe Acrobat Reader and also in Chrome. Seems like Apple needs to update their encrypted reader.
If you actually want to decrypt and use these, you cannot do this with Adobe Acrobat Reader, it insists on keeping the encryption even if the sender just wanted to be safe mailing it. Even if you try to File Print and says “Save as PDF”
The workaround seems to be to use Chrome, you have to use File Open, decrypted, print and then use the “Save as PDF” and it works. Sigh. Complicated!