I want to show aerial imagery as layers in my pdf, so people can turn them on and off (e.g historical images). However according to ESRI images become merged when exported as a layered pdf. What is the point of this? The only way around it would be to merge the tif (or ecw in this case) to a single band image but this would result in it being grayscale I think? I tried everything...
Try overlaying multiple data frames within your .mxd. When you export to layered PDF only images or transparencies from within individual data frames are merged into one image layer. Essentially you can have many imagery layers, rasters etc all in one PDF, all individually toggle-able, but stored in different data frame folders within the PDF. Make sure that your data frame alignments & extents are identical by zooming to the same features or bookmarking in each data frame & using guides around the edges of the document to snap your data frames to.
Another tip is to open your exported PDF in Acrobat Pro, and use the "Reduce File Size" tool. This will reduce an A3 PDF with Aerial Imagery and rasters from aprox. 40 MB to approx. 1 MB.
the only way to accomplish that is to not have any other layers with special symbology.
Starting here in esri help it explains some of the export functionality.
Here is more detail about the pdf export. esri help Pay particular attention to the "rasterization" of certain layers, such as transparencies, and other special label marking.
Click to Export Map from the File Menu in the ArcMap that open a export map dialogue window at the bottom of the window you see tabs for General, FOrmat, Pages and Advanced. Go to the Advanced Tab in the Layers and Attributes drop down select Export PDF Layers and Feature Attributes also check if you want your pdf to have coordinates check export map georeference information box. Then export the map this will enable the layers functionality in PDF. For viewing different layers in the PDF i think you should have acrobat reader 8 and higher version. In the PDF expand the Layers list on the left panel. I've also attached the examples for you. I hope this will solve your problem.
See http://support.esri.com/en/knowledgebase/techarticles/detail/30882. In ArcGIS all layers with rasters, transparency settings or bitmapped symbols, and all layers below them in the TOC get combined into the Image layer in a pdf. If you have Adobe Acrobat Professional 9 or later you might be able to export each image to its own pdf, and then use the Import as Layer menu option in the expanded pdf Layers window to add additional "images".