is it ok to power a 90v dc motor with 170v dc and pwm?
No, it's not very OK.  During any software glitch, you might be running your motor at 125% of rated RPM.  (DC motor speed is proportional to drive voltage.)   Instead, use a single 400V power diode rather than a bridge rectifier.  That should run your motor at roughly 65% of full speed, at most.

90VDC motors are intended for use with motor speed controls:  a specialized triac dimmer followed by a full-wave bridge rectifier.  ($40 to $100 on eBay!)   The little KB Inc. or Dart motor controllers typically adjust from 0% to 75% of full AC output.

Your 170V reading is probably wrong; you're seeing the peak volts rather than unfiltered true-RMS average.   

To reduce the motor voltage, you could try using a standard incandescent lighting dimmer, plus a bridge rectifier.  Choose a dimmer-wattage rating appropriate to the horsepower rating for your motor.

If you want to play with opto-isolated, voltage-signal AC controllers, VELLEMAN KITS has a $30 unit, a lighting dimmer pcb kit with 0V-10Vdc control input.