Releasing free space in mdf file to OS
What exactly you are trying to achieve by shrinking the database? 
You should plan for the database growth and have reserved space for this growth.

So why don't you leave it as it is to have allocated space for new data coming to your DB.

It has been discussed so many times that you do not want to shrink your databases. Check this out and think twice before shrinking:

Stop Shrinking Your Database Files. Seriously. Now. By Brent Ozar