Here is a search engine I have found helpful for Personal Book sources. It doesn't replace other types of searches, but it can be helpful. Just thought I would share it.
PDF eBooks Hub - Free eBooks Database
Still searching!
You wouldn't believe what I forget sometimes!
(To future readers, I forgot to put in the link)
You can also use a Google search with filetype:pdf as one of the search constraints, e.g.,
"holy sonnets" filetype:pdf
