Optimizing pdf files

I would like to optimize a bunch of pdf files, located in various directories.

I tried the pull-down for pdf optimization, but all the options are grey-ed out.

I tried searching your web site, but can find no mention of pdf optimization.

Is this a service to be purchased, and, if so, what will be the charge?

TIA,
Ephraim

https://ww.wp.xz.cn/plugins/ewww-image-optimizer/