Sometimes you have converted a PDF that was a presentation rather than a document. The default scaling mode for PDF is to scale to the width of the viewport which may result in a vertical scrollbar.
For a document that an be split into multiple pages such as a presentation or PDF, there is a new toolbar item called Scaling.
You can now choose which scaling mode the page should use, and optionally apply it to all the pages in the document that was split. For instance you can fit a presentation-style page to the viewport so that no scrollbars are required irrespecitive of the viewport dimensions.