With all the bootstrap being added lately, eventually someone will run across bootstrap not playing well with pdf generation. I was able to get bootstrap tables and most other styles to work with mPDF by creating a bootstrap 3 theme specific to pdf. Some functions, like grids, don't work and I am working to re-style to html5 using width percents. Because bootstrap calculates many styles on the fly(even calculating margins and padding) this may not be possible.
Anyway, Until I get to adding capability to custom report and you run across this, I can give you the theme.