Hm, I try something similar, now in 2021!
Using webP works with pthumb but without Resizer.
But is doesn’t use the parameter for compression, q seems to be only for jpg.
The docs of phpthumb seems to be outdated:
Any clue, anyone?
[[pthumb?
&input=`[[+url]]`
&options=`w=780&sfn=0&f=webP&q=65`
&useResizer=`0`
&toPlaceholder=`bigImage1`
]]
<img src="[[+bigImage1]]" width="780" alt="PDF" />