If you also have the original source files, a lot of your time is saved and makes your job a lot easier. (elearningindustry.com)
This should only happen if the original source file was set up with a large page size (A4 or US letter). (gyrovague.com)
Some vendors, such as Amazon, will convert the ePub to a different format (MOBI, for Amazon) after the fact, but the original source file can start as an ePub. (draft2digital.com)