Use the Print Production tool.
Click More Tools (if you have this)
Click Use Print Production or Print Production
Click Preflight (wait about 2 minutes for it to install if you have never used it and then close and reopen Adobe Acrobat).
At the top of the Preflight window, change the drop down from Essentials to Acrobat Pro DC 2015 Profiles.
Click the Wrench to the right of Default under the Profiles Tab.
Click Default under Profiles and change to Pages.
Scroll down to Scale pages to specified size (Scales all pages of the PDF proportionally to entered value (fit from the inside and add white space)
Click on Edit to the left of the white flag.
In the fixup category you should be on Pages
The Type of Fixup is Scale pages.
Choose the short edge and put in the new size you want. For example, if you’re scaling from 34×22 down to 17×11, the short edge is 11, the long edge is 17. If you want , the short edge is 8.5 and the long edge is 11.
Apply to all pages.
Change unit to Inch.
Enter file name and location and hit save.
Once you do this the first time, the settings should be saved so that all you have to do is go to Preflight and hit Fix.