Exporting for interactive from InDesign. Trying to make the file size smaller. Currently around 18-19MB. Should be around 4-5 where possible. Any advice? There are certain embedded videos, I've tried to keep most as clickable links to URL's.
Thank you in advance
Almost certainly the main factor in the file size is going to be the size of the video files you embed.
Remove all of your videos and compare its size against your current. You will see it's due to these--and InDesign doesn't have a button to make them shrink (where it's usually requested that additionally the quality, frame size, and frame rate ought to remain the same).
The best way to make a PDF smaller is to put less in it. Less text (although that won't amount to much -- but still!), less images, less videos.