A scanned PDF is a picture of a page. Your PDF viewer cannot search or copy from a picture. OCR (optical character recognition) reads the picture and gives you the words.
Scan at 300 dpi if you can. Keep the page straight and well lit. Black text on a plain background reads best. Small print, handwriting, stamps and curved book pages are the usual causes of mistakes.
Recognition works with one language at a time. Choose the language of the text, otherwise accents and special characters come out wrong.
OCR can swap 0 and O, 1 and l, or 5 and S. If the text holds amounts, dates or account numbers, compare them with the original before you use them.
The Xargaar OCR Toolkit runs the recognition in your browser, so a confidential scan never leaves your device. Add the file, pick the language, then copy the text or download a .txt.
Open the free OCR ToolkitThe offline pack is still being tested and is not yet on sale.