We have been publishing a series of articles entitled Understanding the PDF file format. In each article we try to take a specific PDF feature and explain it in simple terms. You can see all the articles so far here.
If you have any suggestions or items, you would like to see, contact us.
IDRsolutions develop a Java PDF library, a PDF forms to HTML5 converter, a PDF to HTML5 or SVG converter and a Java Image Library that doubles as an ImageIO replacement. On the blog our team post about anything interesting they learn about.