Though I must have done it wrong because the PDF I got was an empty file. Maybe someone else can figure it out.

You need a slightly modified command for Umbrel.

Add these to your .bash_aliases and then logout and back in again and they should work. https://gist.githubusercontent.com/derekross/fb74d328863046ff3b262f2d5e59bb09/raw/947cfda48043ae0c750b4247645e07275b19972b/umbrel-aliases.txt