How does the website Coursehero blur their documents?

I analyzed the Coursehero website and saw that they use a blurred image (jpg) as canvas and then text written over it. What would be the best way, to achieve a similar result with my documents - a PDF file for instance, where I want one paragraph to be blurred and the rest visible.

for reference: https://www.coursehero.com/file/13720087/MIcroscopyLabBiologyLAB2/

In the first page, part of the document is blurred and shown as an image and the other part of the document is written over it. What tools could be used?