A scanned PDF comes from a scanner or from photographs, so every page inside is an image. Recognition is required before the content can be extracted.
Scanned PDF
A PDF that contains only page images: the text cannot be selected or searched.
Related articles
termDPI
Dots per inch: the pixel density of a scan or a print.
termOCR
Optical character recognition: turning a picture of text into editable text.
formatPDF
PDF file format: extension, MIME type and available conversion directions.
termPDF text layer
Characters stored alongside the page image — search and copying rely on it.