I would like to develop an app for printing service in Windows Phone 7. In the app, the users can select file to print out. Is there any way to open docx, xlsx, etc. in WP7
If you are looking to open the document in Office, try using the WebBrowserTask to navigate to the document. There's a good chance it will open with Office.
If you want to render the document yourself, you are very much out of luck. You could try navigating to the document in a WebBrowser control, but I don't think it's supported.
Besides, there are no printing APIs so there would be no way to print it. If this is an enterprise application (ie. internal to your company), you might consider writing a web service on a local server that can accept DOCX files and print them. You could then call that service from your app to print the docs.