The PDF specification, first as a published standard and then as an ISO standard has been freely available for years. Any programmer can read the PDF specification and start creating their own PDF API to sell as an SDK, or to power their own PDF software application.
However, the PDF specification is about 1,000 pages long, and also references a range of other standards such as image standards, XML standards, font standards and so on. Add to the mix that some areas of the PDF specification are ambiguous and you can see how ensuring your PDF engine can consume billions of different PDF files could be problematic as every PDF is unique in some way.
Due to these rigorous standards many startups and even larger companies, will first look to a commercially licensed SDK or an open source project.
To read more visit the Foxit SDK Blog Article – PDF and the Edge Case
