Every 2 hours there are direct train services called "Railjet" or rj from Prague (Praha hlavní nádraží) to Vienna (Wien Hauptbahnhof). Most Railjets go till Graz. With this trains you get without change to Vienna. You can reserve a seat via tickets.oebb.at or cd.cz , but it's not compulsory.

You can also try Regiojet (RGJ or RJ). In this trains I think you have to reserve a seat or it is highly recomended. 

Thanks, I am aware of this.  My question surrounds the trains with one stop.  Is it easy to connect with the second train. The time in between trains is very small. Thanks

The change in Breclav of 3minutes is quite tight even if the trains should be next to each other at the platform As @Angelo  said it´s better to catch the direct Railjet or the Regiojet service (the Regiojet service is quite cheap fares from 18€ on the Route )

@johnmlindsay1956  Only reason why still go for the Eurocity Connection with changes are the dinning cars of the trains :D

Thanks, I will book the direct train, John

Dining/eating: then I really recommend the yellow trains of RegioJet-they have quite good (microwave) hot meals. These are even cheaper if you book ´comfort class´=older 1st class cars.

With a pass you can get seats, but only in Bisnis class=without service.

As above: there really is not any reason to go for connecting trains on this line.

If I order on online do I receive a ticket to print on line or do I access on the app

For the Route Prague - Vienna you can get the reservation as online document that can be shown via your smartphone :)
My suggestion reserve via oebb.at (Austrian Rail for 3€ per seat without any service fee ) or via CD.cz(Czech Rail for arround 4€ per seat)