Why search fails
A PDF can look like text without actually containing searchable text.
A PDF created from a word processor usually stores real characters. A scanned PDF can instead contain only page images, so the words are visible but there is nothing reliable for Ctrl+F to search.
OCR can add a text layer to image-based pages. PDFBright checks the document first so you can see which pages already have usable text and which pages are likely OCR candidates.