Often this happens, because the pages in a PDF are added as pictures, and not as text. Often this is the case with scanned pictures from the paper magazines or layouts done in PhotoShop or similar. OCR (Optical Character Recognition) technology can be used to allow the text to be embedded inside the PDF.

Adobe Acrobat provides a simple-to-use OCR engine directly in the Acrobat interface. This article from Adobe explains it's use here

In case you do not have access to OCR technology through Adobe Acrobat or similar, we suggest that you add the missing words by editing page texts in the Pages tab of the "Bookmarks / Pages" dialog e.g. inside eMagStudio (available in the Navigation step of the Wizard).