Struttura di un file PDF

• https://it.wikipedia.org/wiki/Portable_Document_Format

• https://web.archive.org/web/20141010035745/http://gnupdf.org/Introduction_to_PDF

• https://stackoverflow.com/questions/88582/structure-of-a-pdf-file

• https://stackoverflow.com/questions/35628636/the-internal-structure-of-pdf-file

• https://resources.infosecinstitute.com/pdf-file-format-basic-structure/

• https://blog.idrsolutions.com/2013/01/understanding-the-pdf-file-format-overview/#helloworld

• http://www.printmyfolders.com/understanding-pdf

• https://www.oreilly.com/library/view/pdf-explained/9781449321581/ch01.html