In order to properly convert a document to ePub, it must contain "real" text that isn’t rasterized or included within an image. To tell if your text is “real” there are a few things you can do:
If you have a PDF:
If you can select the text separately and copy and paste it, your document contains text that is ready for conversion.
If you have a Word document:
If you can select the text separately or modify it in any way, your document contains text that is ready for conversion. If your book has images that contain text, your final ePub file will also show that text as part of the image.