You mean reducing its size? Try Save As. A document that's has been saved many times over, accumulates size.
Once you've done a Save As, it probably won't get smaller than that. What's the purpose of compressing it?
Exporting the file to IDML (File > Export > Type: InDesign Markup (IDML) may reduce the file further. Opening an IDML in InDesign, in the same release that it was created in, recreates the file completely as an InDesign file. You'd need to save with a new name to keep it as an ID file.
You can try to reduce even further, by compressing the IDML file, with a file compression utility.
How to comress a file from indesign?
You could also use the Preferences
Under File Handling
Turn off the save Preview of pages.