Rummage
Rummage is a web application designed to provide curated lists of pdf documents available on the Internet, and allow levels of search on them.
A list of sources in a database is queried with the Sources button. These range from Internet Archive collections. lists and items, YouTube channels, web pages with lists of PDFs, curated lists of PDFs and even just single PDFs. Hover over the items to see a sample of what they contain. Some sources are static, some automatically update with different refresh frequencies.
You can then search within those selected sources with the Subjects button, and get links to individual files.
The filter box allows you to thin out what is on display dynamically. Unlike the other searches you can include leading and trailing spaces.
Searches came have multiple terms separated by ; and negated with !