Large PDF filesizes can definitely be an issue, as many clients/customers require a PDF that is not huge, so trying to reduce the filesize is an interesting challenge. There are several easy ways to do the reduction, but the trade-off in quality can be a concern.
It is important before starting a reduction to decide what level of quality will be needed for the final product.
Then, the place to start in the quest to reduce filesize is the export settings in ArcMap. There are quite a few, so I'd experiment with them all to see what the tradeoffs are. When doing File, Export Map, be sure to click on the triangle by the word Options at the bottom of the export window to reveal the settings tabs.
Exporting to PDF—Help | ArcGIS for Desktop
If that doesn't go far enough, consider running a file reduction process in Adobe. There are several processes available which will "flatten" a file, but at at cost of quality. Also, most of the processes will greatly limit the ability of downstream users to edit the result, as many of the vectors will be rasterized. Depending upon which Adobe product you have, these processes have different names, so one may have to poke around to find what the process is called.
Optimizing PDFs in Adobe Acrobat Pro
https://acrobatusers.com/tutorials/reducing-file-size
Lastly, in some circumstances one can vastly reduce the filesize by making some adjustments to the layer display in ArcMap. Specifically, if one forgoes using transparency in the layers in the Table of Contents, one can vastly reduce the filesize. The ArcMap export process greatly increased filesize of transparent layers by automatically rasterizing the topmost transparent layer and all layers below them in the Table of Contents. So undo any transparency, or if the transparency is needed, see if the layer can be moved down in the table of contents. Doing so often results in quite a surprising reduction in filesize.
Chris Donohue, GISP
