PDF pixlated after saving from photoshop

Hi there,

I have been using photoshop to edit a pdf for the last 3 years. I have never had an issue with my pdf becoming pixelated when I save it after editing on photoshop. I started facing this issue about a month ago when I tried to edit my pdf and then saved it. 

Please have a look at the attached photo to understand my issue. Photo 1 is a screenshot of the direct pdf from AutoCAD, photo 2 is that saved from photoshop.
So far I have tried to open the pdf at 600 pixels/inch, bit depth at 16, even tried to get a previous version but it just doesn't work. If you have some useful tips for me, I would be very thankful!

 For what bizarre reason are you using photoshop to edit a pdf?!?

I study architecture. I use AutoCAD to produce pdfs of my plans, elevations, sections etc. At university, we are taught to use photoshop for post-production. I am aware that Photoshop is not the best for post-production of pdf where we normally add textures, colours, etc. I would have taken your suggestions and learned illustrator but I am in my final year, I am stressed, I have a portfolio due very soon, and I am just a very tired uni student amidst this pandemic. I have been using photoshop for the last 3 years and I can edit my plans, etc in under 2 hours. Trying to learn illustrator now will take me a while.