A good tool for you can be google search.
I find two documentation pdf file about marshmallow about with this tool:
Use this to find the PDF files into the website:
site:https://buildmedia.readthedocs.org/media/ + filetype:pdf
A complex search can write in this way to find search by books and comics depend by file extensions:
-inurl:htm -inurl:html intitle:"index of" "Last modified" books epub
-inurl:htm -inurl:html intitle:"index of" "Last modified" comics cbz
This will be