When I run the executable \"wkhtmltopdf-amd64\" (from Linux), I get the following output:
QPainter::begin(): Returned false============================] 100%
I know I'm late to the game here, but I was having this same issue. My issue was the fact that I was trying to write to a directory that didn't exist.
I had assumed that wkhtmltopdf would create the directories that I passed it. It did not.
Making sure the directories existed before trying to create my pdf solved it.
I hope this helps someone else.