Why do my CAD drawings turn blurry once they go into a Google Slides deck?

Yeah, Google Slides is recompressing the image on upload, and the bigger the source image the harder it mangles it. Your PNG is fine on your machine, but the moment it goes into the deck, Slides downscales it to fit its internal processing and then serves up a soft, blocky version. That's why bumping your export resolution did nothing. You just handed it an even bigger file to crush.

The fix that's worked for me is to stop exporting a giant D-size sheet and instead make the image roughly the size it'll acutally appear on the slide. If the drawing is going to fill a quarter of a slide, you don't need a huge raster. Plot to a smaller sheet size, or crop the PDF down to just the area that matters before you make the PNG. When the image dimensions are close to the display size, Slides has way less reason to chew it up.

Two other things that help a lot. First, fatten your lineweights before you plot. Thin lines look crisp in a PDF at full zoom, but once they get rasterized and then shrunk, they basically vanish or turn to mush. Heavier lines survive the trip. Second, skip the PDF step entirely if you can. AutoCAD can plot straight to PNG or JPEG, and one less conversion means one less place for quality to leak out. Breaking the drawing into sections and inserting them separately can work too, since each piece ends up at a more reasonable pixel count, but honestly I'd try the smaller sheet size and thicker lines first. It's a five minute change on your end and usally solves it. If a slide still looks rough, another cheap trick is to just link the full PDF next to the fuzzy image so people can open the real drawing if they need to read the fine print.

This explains a lot, because I had basically the same thing happen with a floor plan I sent to my brother. On my laptop it looked fine, then he projected it at a meeting and everyone squinted at it like it was a fax from 1995. One thing I noticed: if I zoom the slide view out a bit, the picture looks way better, so I wonder if it's also about how big the picture is stretched on the slide itself. Does shrinking the picture down on the slide after inserting it help at all, or is the damage alredy done the second it uploads?

I ran into something similar with a survey sketch I made for a fencing quote, though mine went through a word processor instead of slides. What seemed to matter for me was not resizing the picture after inserting it. If I dragged the corners to shrink it down inside the document, it got blurry, but if I cropped and sized it properly before inserting, it stayed decent. No idea if that's the same problem you're having, but is your coworker stretching the drawings to fit once they're in the deck? Might be worth checking before you change anything on your end.

One thing that helped me when I had to put a drainage detail into a client deck was plotting the PNG straight out of AutoCAD at 300 dpi instead of letting some intermediate step guess at it. The plot dialog lets you set that yourself, and the difference was night and day compared to making a PDF and grabbing an image from it. Also keep in mind the deck only has to look good on whatever screen it gets shown on. Ours is a 1920x1080 projector, so anything way past that pixel count is just getting thrown away anyway.