You need to set up Data Driven Pages. This is a good source for information on that: http://desktop.arcgis.com/en/arcmap/10.3/map/page-layouts/creating-data-driven-pages.htm
Then, you can do the following steps:
1 Verify that Data Driven Pages are enabled and that ArcMap is in layout view.
2 Click File > Export Map from the main menu.
3 Navigate to where you want to save the export file.
4 Click the Save as type arrow and click PDF.
5 Type a file name for the export file.
6 Click the Pages tab.
7 Choose what pages you want to export.
8 Click Save.