PDF is a format that looks like the original book. The advantage of this format is that we can produce it quickly; nevertheless, it may not be suitable for complex texts, such as mathematics or physics.
Typically we produce text readable PDFs by chopping and scanning a book, though sometimes we are able to get the digital publisher file. OCR (optical character recognition) makes it possible for assistive technology to read the file. A PDF can be used with various types of assistive technology including Kurzweil, ZoomText, text-to-speech software and screen readers.
It is the fastest format for us to produce. On average it takes 5 days for us to produce a PDF.