To view two documents side by side in horizontal or vertical in the same Foxit PDF Reader window, please use the "New Horizontal Tab Group" or "New Vertical Tab Group" feature which was implemented in Foxit PDF Reader V12 and greater versions.
Firstly, please open two PDF files in multiple tabs in the same Foxit PDF Reader window. Secondly, please right-click a file tab and choose "New Horizontal Tab Group" or "New Vertical Tab Group" in the pop-up context menu to display the document side by side.
When the documents are displayed side by side in the same Foxit PDF Reader window, you could scroll the document in each document area without affecting the other document view. In each document view, other than viewing the document, other operations also can be performed, eg. viewing bookmark/attachment pane, making annotations, etc.
