@samo79
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The engine is opensource but not the GUI around it
Not possible with GPL. If you use GPL code in a program and distribute binaries you're required to also make available the source to build those binaries. Read the license. It could be possible if engine and GUI were separate binaries but they aren't in AmiPDF. The only way that's not required if you have a commercial license from the copyright holder of xpdf who can license the code under different licenses but if they used the GPL version then all of it is under GPL including the code they added or changed.
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Engine was not updated aniway, it's the same as the old version from Hyperion
Was that because the commercial license was only for that version? That's a bad idea as according to
https://en.wikipedia.org/wiki/Xpdf AmiPDF was based on xpdf 3 which is probably missing a lot of bug and security fixes so another reason to publish source so it can be updated with current GPL xpdf that fixes vulnerabilites and allow other people to support it when the original author does not have resources for that any more.