Atticus can sometimes import ePub files, but it really depends on how the ePub was originally created. Different programs use different ways to organize information in ePubs, which means that Atticus may not always be able to extract the necessary information from the file.
Even if ePubs import successfully, none of the old formatting will be kept, and things can get messy because conflicting coding from the original ePub file can sometimes stick around, leading to formatting or validation issues down the line.
For the best results, we strongly recommend importing the original source file in .docx format. Atticus works much better with .docx files because they have a more consistent structure. This way, you'll have a smoother experience with Atticus and avoid a lot of formatting problems.
If you don't have the original source file, you can try using a program like Calibre to change the ePub file into .Docx format. After that, you might need to tidy up the file a bit. We also suggest following this tutorial to ensure your document is ready for a smooth import: https://www.atticus.io/how-to-prepare-your-word-document-for-upload/