Even though they may look the same on the surface, PDF files are different. That is, they can be created in different ways. One way to create a PDF is by converting an electronically generated document that was created by using programs such as Word or Excel.
Another way is by scanning text or images from paper documents.
Creating scanned PDF files, or image PDFs (so-called because the resulting PDF content is only an image of the document) is, by far, the number one solution for transferring paper files into digital ones.
When you scan a document into the PDF format, you will need a quality scan to PDF document software tool. Fortunately, Adobe Acrobat 11 Pro is just one of many PDF scanning software tools that can get your documents into a digital format quickly.
Note that a scanned document is only a picture of the text or image. As such, you will not be able to perform a word search on it. To do so, you will need to run it through an Optical Character Recognition (OCR) engine found in most OCR enabled PDF converters.