Are you viewing the image in "Typical" or "High Quality" in your display options?
Right Click / Display Performance (at the bottom)... make sure it's on high quality display
Take two or three steps back.
We're not familair with how you created the pdf from the AutoCad, either via Printing to Adobe PDF or the software has pdf creation built in.
In either event, that original pdf should not have been opened in Photoshop. Photoshop would have rasterized the type, no longer actual vector type but pixel based, dependent on the resolution when opening in PS.
Was there a need for Photoshop?
What about just opening the PDF in Acrobat instead of Photoshop. Is it clear in Acrobat?