I copied the contents of wp-content/plugins/wp-mpdf/themes/ to wp-content/wp-mpdf-themes/
This has given me the the ability to select "default" as a theme in the settings page. The installation instructions says something about some theme files, which is guess is what this is about, but it was not very clear at all. I only found out about this by looking at the code.
It is now telling me that the cache directories are not writeable (I am only seeing this with debug turned on, otherwise I would be totally in the dark). Now, I *did* set the cache folders to be writeable, but installing the last update has reset those folders.
Two things I would suggest may help:
1. Error reporting. Tell the user (or admin) that something is wrong when a folder cannot be written to, rather than just dump them into a while screen.
2. Why no set the folder permissions during the install and upgrade process? If permissions cannot be set, then give the admin the instructions on what folders to change permissions on. There are three (or perhaps four) folders that need changing, so make it easy for the admin rather than having to go on a bug hunt and digging through documentation *every* time the plugin gets updated.
It looks a great plugin, and it is much appreciated, and I'm just playing devil's advocate here - I would rather tell you what I think is wrong, and what would probably make a lot of other people give up long ago. Obviously take it or leave it, but all comments meant with the best intentions.