how do I put my apps on my pc desktop?

You can have them on your computer's iTunes for syncing and for making back-ups, but if you are asking whether you can use them on a PC then no, apps from the iTunes app store can only be used on iOS devices (iPads, iPhones, iPod Touches)

You should be able to copy it from the iPad to your computer's iTunes via File > Transfer Purchases, and (if you want it on the iPhone) you can then try syncing it to it - some apps are designed to only work on the iPad (i.e. they've been designed for the iPad's larger screen), so it may not necessarily sync to the iPhone (the left-hand side of an app's description page in the store should say what it works on).

Apps (and other content depending upon which country that you are in) can also be re-downloaded from the store for free (as long as they remain available in the store) : re-downloading.