This question has been asking repeatedly, so I think I should share it here. The question is: how can we have layer information when we plot to PDF files? The same question is for DWF format. PDF probably is the most popular format for sending documents because PDF reader/viewer is free. The problem is, we still want to have control to our drawings. One of the popular information we want to keep is layer information.

Choose DWG to PDF plotter
When you plot your drawing, choose DWG to PDF plotter. It’s included in AutoCAD installation, so you don’t need any other additional PDF plotter. It is quite good, and actually I think it works better for AutoCAD.
Click properties on the right side of plotter name.

Configuring plotter
In plotter configuration dialog, choose custom properties in the list. Then click custom properties… to make necessary changes.

Custom properties dialog will open. Make sure include layer information in this dialog is checked.

Now click OK until you get back to plot dialog box. Try to plot your drawing.
For DWF plotter
If you want to plot to DWF file and have layer information, you can do the same steps as described above.