This can be accomplished with Acrobat Professional. I am not certain if Acrobat Standard can modify page sizes, it might.
Screen shots are from AA 9 Pro. Study. Both AA Standard and AA Pro install the Adobe PDF printer (as shown).
Print > Printer = Adobe PDF Page Scaling > Shrink to Printable Area
Properties - Set Paper size to less than Letter
My resultant pdf page size is 7.5 x 10. Note the white space at the top & bottom in the Preview pane.
I have opened (my) Pages Panel, right clicked a thumbnail and choose (the un-intuitive) Crop Pages to increase the page size.
Resultant PDF with white space
