Compression PDFs

"Never, never, never downsample images to 72 dpi!" Well, it depends what you want. If I want a rough layout I go down to 50 ppi or less. Also, for photos (not screen shots) it's a myth that you can only get acceptable results if image resolution is at least screen resolution. Try it! 

And if size is the priority, then we must be prepared to compromise quality. 

I would continue to recommend 72 ppi for almost all work intended for screen viewing. I would advise against using anything over 100 ppi, it just makes a needlessly large file no matter what nominal screen resolution.

I kind of have the same question as the OP.  I understand your response that the two tools don't have the same compression parameters.  I have Acrobat X.  I have compressed a ~6MB file using the Acrobat online tool (as a one-off, as I don't have the paid cloud version).  I used Medium and Highest compression, resulting in files of 2.3 MD and 268 kB.  When I use Save As Reduced, or Optimized through Acrobat X, I get literally no reduction of filesize.  While the compression parameters may not be exactly the same, I can't imagine they are that wildly different.  Is there a way in Adobe X to emulate the compressions done via the online tool?