I wouldn't call it a "replacement", perhaps "alternative" is more suitable. AEM Guides allows you to author content in DITA XML i.e. ditamaps, topics, images (inc video).
You can then publish your DITA content to a wide variety of outputs:
- PDF using either Adobe's Native PDF generator or the DITA Open Toolkit (DITA-OT). There are default stylesheets, but most will customise.
- HTML
- Webhelp
- Epub
- AEM Sites i.e. direct to a publish instance of AEM
And many more.
AEM Guides includes an excellent web editor to allow your team to authors their content in the web browser or you can use desktop Adobe FrameMaker or oxYgen XML Author.
There is a Word2Dita conversion available, but it is basic and the DITA would probably need tweaking after conversion. If the topics are fairly small in size you can copy and paste from Word into the AEM Web Editor or oXygen.
DITA supports "resource files". These are file types (Word, Excel, PDF) that are not native DITA, but can be included in your outputs.