I can't remeber Word95 that well, but on Word2000, in the print options, there is a 'zoom' box with a 'Scale to paper size' option that you can set to A5. Send that to the PDF printer and that should be that, I think.

I am a bit confused by the 'centered' remark, though. If it's a PDF then surely they want the PDF to be 'A5' pages (as the above would create). I can't think they really want the A4 pages reduced to A5 and then centered within an A4 sheet within a PDF document. But who knows?