This only works with some PDFs in actual text format, not PDFs which are page images of text. The anon supplied one example where it does work, A defense filing today by Paul Manafort's lawyers:

https://assets.documentcloud.org/documents/5677512/Manafort-20190108-Dc.pdf

I have verified this myself. The unredacted version is not that startling, and I'm not going to post it right now. Some anons say that if the redaction had been done properly, this wouldn't work. I don't know.

However, you might want to look for other more interesting documents to try this on, as many 8chan anons are now doing. I suggest copies that have been downloaded and re-saved on archiving sites such as scribd, documentcloud, Google drive,etc., or that you have saved yourself in the past. The documents in their original locations might have been modified (or might soon be modified!) so that this doesn't work.