The Combine function is reducing the quality of my images signficantly

I create a series of scan ned text images at 225 dpi.

When I add the image to Acrobat and combine it into a document, the quality of the image deteriorates dramatically.

on the left in the image below is the original image at 400%. on the right is the image after combining it in an Acrobat document (no other work done yet). Note that it has degrated dramatically.

What causes this? How do I fix it?

Thanks!