x86 programs run perfectly (and start faster) on x64 system. Only thing not available is addressing over 2GB of memory.
Since in 2017 we are adding our own format where you can use more than 5-6 GB of memory, realistically, when needed, we are also adding full x64 version of PA.
Also, codecs built from grounds up might support faster instructions in x64, so with .pa format you will see up to 30% faster performance in x64 than in x86… for .zip, performance difference is nowhere as close, if any.