Master PDF Editor is a free tool for personal use on Linux systems, it is not open source, but is an option to consider for the tasks involving editing and modifying pdf files.
It is a complete solution for creating and editing both PDF and XPS files, and lets you add content to existing PDF and XPS files. You can add, edit, import, add images. You can also add text, or edit it. Similarly, you can add or remove pages, change the format of the pages, and of course change the PDF information, author, title, subject, keywords and other information about the document.
Besides these features, you can also encrypt and convert PDF files to XPS format.
Another interesting feature of this application is that it lets you create and edit forms, from buttons, combo boxes, list boxes, text entry boxes and more.