First off, don't export as a PDF to open in Illustrator, export as an AI file. If you that still crashes Illustrator you could try exporting the data by layer, rather than all at once (or layer groups). Open these layers in Illustrator separately, use the release clipping masks command to get rid of the Arc masks (you will need to actually select the empty masks and delete them as well). Also use the path>simplify tool to reduce the number of vertices in your lines. If you need the the masks on individual features either re-establish the mask in Illy or treat those layers separate from the rest. Re-combine the layers and save as a PDF from Illy, or save as EPS and use Adobe Distiller to create a optimized (smaller) PDF.
Thanks for the tips! Well, of course I export as AI when I do vectors.
Why release the clipping masks? I need the masking, but I'm not sure which masking method in Illustrator that is the best? There are perhaps 10 layers that uses masks and one is contour lines so the mask is very complex.
In the end when I tried exporting the PDF it runs out of memory. Seems like the old version of my PDF:s 1.3 can't handle that size of files? (I need that version for the printers...)
Sorry, I misunderstood your original post and thought you were opening the PDF in Illy.
From my experience, Arc creates many unnecessary clipping masks on individual features that could be handled as a single mask around your frame. All of the clipping masks from Arc will be rectangles, so none will be more complex that the others, they may just clip more data, or use multiple masks on a single feature layer. Releasing the Arc masks and re-establishing them in Illy allows you more control over what gets masked, where and with how many masks.
Also be aware (you may already know this), that if any layer being exported to PDF has a transparency or raster in it, all layers will be converted to raster, greatly increasing the file size. Turn off transparencies and export rasters separately. Re combine in Illy. If you want to go straight to PDF, try the EPS to Distiller route for smaller PDF file sizes.
Also, I recall an ESRI forum message about who to reduce your PDF export file size by making a change to the Arc preferences but not sure what the details were. You my want to look into the default PDF export settings in Arc.
Not sure if any of that will help, but good luck!