the PDF format is a meta format which may contain raster data as well as vector data.
As @Anssi already elaborated, under macOS PDFs are imported by using the graphics engine of the macOS which supports raster data only even if the PDF contains vector data… pretty useless.
If the PDF contains vectors (simply try to select an entity w/ Adobe Reader for checkking this) and if you need vectors, open with an graphics application like Illustrator, Corel or free InkScape, export to DWG format and import in SU.
OK so this is odd…
Working on a job inserting pdf details created from Layout and one of them is rendering as a vector…
No matter how far you zoom in the image is crisp…unlike the others.
For certain some pdfs inserted into Layout render as vector rather than raster. It seems random and I haven’t been able to figure out what triggers it…