Although it is possible to split your diagram over multiple pages on export, there is no way to specify what part of the diagram will be on which page.
If you want to try automatic splitting nonetheless, disable option "Automatic Page Settings" on tab "PDF" and enable option "Activate Tiling" on tab "Tiling" in the PDF Export settings.
If you absolutely need full control on which part of the diagram will be on which page, you will have to export each page separately with option "Region" on tab "Clipping" set to "Visible Region". Of course, this also means that you need to adjust the view port of the editor such that only the part of the diagram you want to export is visible.