I tried to use \pdfminorversion=7. This did not work, the error "!Undefined control sequence" popped up. Just to see if it would work, I also entered \pdfminorversion=2, with the same effect. Is there a possibility to create PDF version 1.7 with texlive 2017 using XeLaTeX? Are there packages or something else needed in addition? Due to a quite complex system that would not work otherwise, I have no possibility to update texlive or use something else instead of XeLaTex.