Documentation

[MCcoder52]

Documentation is spotty and seems to be expressed solely through examples. It is a great tool but if you don't search blogs and forums, you won't learn that page size is expressed in millimeters (for example). I assume that is true of margins as well. Good docs would preclude the need for assumptions.
I understand the difficulty of maintaining documentation for a changing tool.
If there is good documentation out there, it isn't being found by Google (Rotativa documentation)

[ArrowRaider]

How did you even find that margins are in millimeters?

[ArjanKw]

The documentation can be improved. All options are documented in code however:

Documentation of all options in wkhtmltopdf can be found here.