As far as I know, there's no .pdf or online copy of any manual before 1994 for the Q45. Any 90-92 manual should be essentially the same... the '94 is fine if you're dealing with mechanical parts like taking the plenum off, brakes, suspension, etc. - but the electronics are substantially different.
If you absolutely want the right book for your car, $50 isn't too bad.
http://www.ebay.com/itm/1993-INFINITI-Q ... 54&vxp=mtr
Every Q came with one in the trunk, so they're pretty abundant on ebay... you might be able to wait and get a better deal. Be sure to check your trunk and that compartment between the rear speakers to make sure you don't already have one.
Heath