Why is last years 2015 return 188 pages when I try to print to file?

The PDF includes your tax return and all the worksheets and other information that is for your use only.
Just select the Form 1040 and any other forms and schedules you want to print by page number when using Adobe Reader.  You do not have to print the entire PDF.

But I don't know which pages I need or not.

Scroll thorough and print the Federal pages with OMB nos in the upper right corner.