combine pages

This is best done in the original application, if this is not possible, you can view two pages in Acrobat as a spread if your page display settings are set to "Two Page View", you can also set the pdf document properties initial view setting to "Two-UP" (File> Document Properties> Initial View), however some pdf viewers will ignore this setting and the setting can be overridden in the Acrobat page display preferences. You can't easily combine single pages into a spread page in Acrobat, AFAIK, but you can place the pages into an InDesign page at the desired size and export to a new pdf.