jee7

I see. Thanks for your explanation. Yes, this is what I meant. First add the white rectangle, which is a bit bigger than the artboard. Then, secondly, export using either the "flatten" or "press ready" presets to merge the layers.

Glad to see my answer has helped at least a few people solve this issue. I should perhaps clarify that the white rectangle I used was a bit larger than the artboard itself. Thus the rectangle itself did not create these lines. Also, did you try the different PDF export options? The preset called flatten will both merge the layers and rasterize. It looks like a preset called press ready will only merge the layers and will not rasterize (you can see the configurations in the More... PDF export settings). I'm thinking that if you use a preset like press ready, it should merge your layers automatically and thus save you the trouble of doing it manually. But I haven't tried it myself.

I had the same issue and figured out the solution. The solution is to put a white rectangle behind all the graphics. Just specifying that the artboard's background color is white, does not work and produces these infinitely thin gray artifact lines around objects. The suggestion to not rasterize is irrelevant. There could be many reasons to rasterize, and as there is an option for it on the PDF export menu, it should work. My reason was that some overlayed graphics did not get rasterized correctly in the printer. I had an orange rectangle partly behind a completely opaque black rectangle. In the vector PDF, it was fine, but in print, the orange rectangle was visible from behind the black one. Another reason is that if you have some big vector files, the printer (or even the PDF reader) might not be able to load all the layers. I have had issues, where some vector layers do not load or are shown incorrectly in the PDF when the files get too big.