Portable Document Format (PDF) is a file format created by Adobe Systems in 1993 for open document exchange. PDF was officially released as an open standard on July 1, 2008, and published by the International Organization for Standardization.
There are three types of PDF files:
Normal, or Pure, PDF – This is the most common type of PDF and is created by conversion tools such as Adobe Acrobat or Nuance PDF Converter. This type of PDF contains the full text of the page with appropriate coding to provide an accurate print of the original document.
Image-only PDF – This PDF will be created from one or more images usually as a result of scanning a document directly to PDF or by converting a TIFF image to PDF. This type of PDF does not contain any searchable text.
Searchable PDF – A searchable PDF is an image PDF that additionally contains a hidden layer of text generated by an OCR engine. This enables the file to be searched in the same fashion as the normal PDF.