Is there any setting you can use to adjust the thickness of the lines in printed drawings from ProE?
Pen tables is the typical method to control line weights for plotting.
There are several useful discussions here on the forum regarding this.
This is also a good link if you have access:
Refer to this conversation about thickening/thinning lines: http://communities.ptc.com/message/173175#173175
Also, refer to the attached documentation to create and activate a pen table file. I believe all the information you need is in the attached documents.