This can have several reasons. PDF files created by scanning are initially built up only by pixels. A person can read the texts, the computer cannot find them at first. Scanners often already have an integrated OCR. However, these may be inefficient. CAD systems often represent the screen display of texts in the output only by line drawings. This happens when the CAD system does not work with standard fonts. The special fonts for the screen maybe are not available in the screen output. Image parts in the PDF can themselves contain text again, which you want to find and recognized..