After the conversion from any other format (Visio, AML or just another BPMN diagram) it is necessary to change the flow orientation of the BPD.
The following example was created from an external source (AML file) and has a “top–down” orientation after conversion to BPMN. The following steps will show you how to change the flow orientation in order to obtain a “horizontal” flow.

Use the Process menu to change the layout:

The following dialog will be displayed:

Finally, you will have to change the page size by using the Visio menu “File -> Page setup…”.