Overview
A scanned PDF is a stack of pictures. Search, copy, and screen readers do not work until OCR writes a text layer. This tool runs recognition in the browser so passport scans and handwritten ish letters do not go to a cloud OCR farm.
Expect strong results on clean printed Latin text and weaker results on stamps, cursive, and low contrast photocopies. Always proofread amounts and names.
If you only have a photo of one page, the Image to Text tool is simpler. Use PDF OCR when you have many pages already bound as a PDF.