Do you have Acrobat 8 Pro? If so use Advanced > PDF Optimizer (think it was there in that version).
Just Acrobat 8.
I found the answer in - Documents - reduce file size - specify which version..
Used to be in 'Save as' in Acrobat X but couldn't upgrade to that in Windows 10 on a new computer...
So, Acrobat 8 Standard (there are two versions, Standard and Pro). Yes, this may work, but only if you can tolerate the images and other things being reduced in file size.