I used hardware solutions like the Fujitsu iX500 ScanSnap scanner for on the fly OCR while scanning and I also used Foxit PhantomPDF to add text (OCR) to images in a PDF.
Both solutions resulted in not bad OCR but there are some flaws like false positives on non text or 99% of the text being missed on a page because there are 3-4 words in the corner of the page that are rotated 90 degrees in another direction as all the other text on the page.
At least PhantomPDF allow you to reviews what the software think might be false positive but you don't have much control.
What are the state of the art OCR solutions to either convert images to searchable PDFs or to add (hidden) text to existing PDFs only containing images (add search feature)?