Open Source Complement to PDF?

nodvin asks: "Is there an Open Source alternative to PDF files? In the late 80's and early 90's I was building and distributing documents in a competing format called DigitaPaper by a company called Common Ground. DigitalPaper was a nice format and more cost effective than Adobe Acrobat. Common Ground seems to have lost out to Adobe (marketing muscle can be more important than the capabilities or qualities of competing products) and the company, or at least the product and format, seems to have been acquired by Hummingbird. Hummingbird is no longer providing any support for the product but is still providing the DigitalPaper viewer and there is a free Common Ground Internet Edition. Perhaps Hummingbird could be convinced to Open Source the code to Common Ground as well as the format of DigitalPaper?"