You need to find an input such that when it is XOR encrypted with an 8 byte key, you get the desired output. The output buffer is generated by calling ASCIIHexDecode() and LZWDecode() on an encoded string.
Since Postscript is a stack based language, you can use the ghostscript interpreter to debug it.
http://www.pwncode.club/2018/11/postfuscator-square-ctf-2018.html