Portable Document Format — Википедия. PDF Specifications, including the PDF Reference for PDF 1.7, PDF 1.6 (ISBN 0-321-30474-8), PDF 1.5, PDF 1.4 (ISBN 0-201-75839-3), PDF 1.3 (ISBN 0-201-61588-6) Portable Document Format в каталоге ссылок Open Directory Project (dmoz)