I don’t remember where I got this, I just jotted it down in my notes.

gs -o inverted.pdf -sDEVICE=pdfwrite -c “{1 exch sub}{1 exch sub}{1 exch sub}{1 exch sub} setcolortransfer” -f original.pdf

Edit: this might well be the original post I found: https://stackoverflow.com/a/30287097

The Old Reader supports 100 feeds on the free tier. And they have a nice list of apps that support syncing to your account:

https://www.theoldreader.com/en/apps/

Might work for you if you don’t syndicate many feeds.