I've downloaded a paid app for iPad (WiFi Map Pro Offline free map) in my laptop but it didn't transfer to my iPad.

Have you turned on Automatic Downloads? Have you synced your device with your iTunes Library?

Perhaps you could try this to resolve your issue: http://support.apple.com/kb/HT4539

You can always go to the App Store on your iPad, as long as it is using the same Apple ID as your laptop, and download the app directly to the iPad.

Oh, by the way, the app you downloaded on your laptop is an iPad app isn't it?