Hi Sara,
Zooming into a map within a map view will change the scale of the map allowing you to see more detail. You can see the current scale of the map at the bottom of the view window.
When you add a map to a page layout you are really adding a container for the map called a Map Frame. You can activate the map frame and zoom in much like you would if you viewed the map with a map view. You should see similar results.
However, and this is the important point, without an active Map Frame you are essentially zooming into the layout page. Much like taking a magnifying glass to a printed map. The map's scale does not change so you will not see more detail like zooming in a map view or activated Map Frame. You just see more detail in the existing display.
I don't believe PDFs have a way of changing the scale or extent of a Map Frame so viewing a map in PDF will be limited to what features are displayed at the map scale present when the layout was exported.
You might want to consider exporting a bookmark map series to PDF where the you determine the map scale for each page so that the user can see more detailed views of your map. Though this does not give your users the ability to interactively zoom in and out, it is an option that might serve your needs. If interactively is a perquisite you will need to find an that your users can use that provide this ability, perhaps consider publishing a web map if you have an AGOL account?
Hope this helps,
Tom
