The ReportViewer control's export functionality can be temperamental.

In order to guarantee that the exported file will not be wider than a page, use the following formula:

Body.Size.Width + Report.Margins.Left + Report.Margins.Right <= Report.PageSize.Width

(since, in our case, InteractiveSize and PageSize are the same, this means InteractiveSize can be substituted in PageSize above)

Sometimes the horizontal scrollbar renders so to bring it back into view you need to add this style:

Then it will show up!