There are many apps that have search over the names of the files but not all content in the files. It is very slow to test all apps such as iAnnotate, PDF Expert, Amazing PDF Expert and GoodReader. Which app search over all content in PDFs?
For non-PDF files and files without OCR, please, see
Search over different types of documents such as DjVU sometimes without OCR?
Best Answer
iAnnotate had the ability to search in all the pdf's That are in it's library (easily import from for instance Dropbox). It's also fairly fast in searching