You should print the autocad plans to EPS using your preferred lignweights. Then open the EPS file in Illustrator. This will give you crisp vector format objects that you can then save/export however you want from Illustrator.

you can import acad directly into illustrator... why print as EPS?

.eps works well.

also, when you save the .dwg in autocad, the CURRENT layer states (what's hidden/frozen/on-off, & what's not) is saved and imported into illustrator as such. If layer "X" in autocad is hidden when saved, it is not placed into the Illustrator workspace. If layer "X" is visible, then it IS placed into Illustrator.

In order to preserve the layer structure, I will sequentially save the autocad file in between placing it into Illustrator with the desired layers on or off... before the desired layer in the .dwg is placed into illustrator, make a new layer in Illustrator and put the freshly imported objects there... rinse and repeat and you'll have an Illustrator file that mirrors your Autocad layer structure.

...doing this allows you to select an entire layer at a time and tweak line weights accordingly.

you can bring dwgs directly into illustrator. so what you can do is set each lineweight to a unique color, then when you paste the dwg into illustrator, click a line, then select similar color, then set the color/lineweight for all lines of that color.

you can do this for all the unique line colors that you brought in from cad.

Don't you experience problems with curves being too faceted?

...adjust the display resolution in Acad

options>display tab>display resolution settings

The actual answer to this question is that Acrobat just simplifies to speed up its display. Go to preferences and change your settings; the information on line weights is still there.

Thanks a lot jesaku, I always wondered why Acrobat does that! It's ruined many a presentation at my school.

"In order to preserve the layer structure, I will sequentially save the autocad file in between placing it into Illustrator with the desired layers on or off... " NOT NECESSARY!

You can open your .dwg (without printing .eps file) in Rhino and you should have all your layers there. Once you import the drawing as an AI file you should see all the layers once you open the file in Illustrator.

so...Autocad>Rhino>Illustrator

I meant that you have to export your drawing from Rhino as an AI file...

Wow all great suggestions...I'm gonna work on trying every one of them. Thanks so much!

I do have one more question concerning jesaku's reply...excatly which preference setting should I tinker with? I've been playing around with that and don't seem to find the right category for the lineweight display.

Go to Edit>Preferences>Page Display. Then uncheck "enhance thin lines" And it should solve your problem...it fixed mine.

If you're using Illustrator CS3 and your acad layers are being removed or condensed into one illustrator layer, you need to download the UPDATED plugin so that it maintains the layers when the file is imported. They (adobe) changed this from cs2 to cs3, it used to be layers came in automagically, then in cs3 they didn't until people complained and they changed the plugin. I don't have time to search for it now but its buried somewhere on the adobe site. download it and replace the current importer plugin with it and all should be well... acad dwg's can come straight into illustrator with all layers intact.

Found it!

http://kb.adobe.com/selfservice/viewContent.do?externalId=kb402977&sliceId=1

thanks Pod!

What version do you use?
I'm using Ai CC 2018, and the issue still exists,
I check the link that provided by @PodZilla but I didn't find a download button and it says for CS3...

can someone help with this?
I wan't to import .dwg to Ai with its layers.