Screenshots look weird on a mac, but good on print

Hey guy's I am making a infosheet for the company I work at as an intern. When I make screenshots of the website, they look good in the Mac previewer app, but when I place them in InDesign, and export a PDF, It looks terrible! When it's printed, it looks good again, but since the infosheet will be e-mailed, the screenshots need to look HQ on a screen.

This is a screenshot of the website

This is the same screenshot, but made from the PDF

I hope there is someone who can help me!

Thanks in advance!

`Kevin