How Does a PDF Store Text

Question

I am attempting to gain a better understanding of how a PDF stores text. Generally speaking, when a PDF is created from an application like MS Word (or in my case SQL Server Reporting Services), how is text stored by the PDF? I would hope that the resulting document isn't OCR'ed in this particular scenario the way it would be if the original PDF document had been created from an image.

To get a bit more detailed, I am trying to understand how text extractors for PDFs work. My initial understanding of PDF was that it stored (PostScript) instructions on how to draw the "image" of the document to a page or a printer, and that there was no actual text contained within the document itself. Subsequently, I was thinking that a text extractor might reverse-engineer such instructions to generate the text that the PDF would otherwise generate. I am not confident of this, though.

Answer 1

PDF contains several different types of objects; not only vectorial or raster drawing instructions. Text in in particular is represented by text elements. These include a string of characters that should be drawn at certain positions using a specific font.

Text extraction from PDFs can be a complicated affair because the file format is oriented for page layout. A text element may be an entire paragraph, or a single character. Even a single word may consist of several text elements if different typefaces are mixed. Also, the characters are not necessarily encoded in a standard encoding such as Unicode. They may be encoded in a way specific to a particular font.

If you are lucky enough to deal with Tagged PDF files such as PDF/A or PDF/UA, text extraction can be a lot easier because text spans are identified as such, and a mapping to Unicode characters is defined.

Wikipedia doesn't have the complete specification but does serve as an introduction: http://en.wikipedia.org/wiki/Portable_Document_Format#Text