What extent of docx support is important, do you feel? Is just importing docx to html (the native format) sufficient, or should they stay as docx while editing? Should it be possible to export from html to docx?
As an end user, I don't care what format Airborn uses internally. What matters is that I can interact with documents and files sent to me by other people, which are likely authored using Microsoft Office, Libreoffice, and Google docs.
As a developer, I would ideally recommend to look into using a custom internal representation for a document and developing converters that convert between your representation and various formats. This way you won't be hindered by limitations of a specific format.
However, if you lack the resources to develop and maintain these converters yourself, look into the feasibility of leveraging LibreOffice by using a preexisting format like WordProcessingML as an internal format. You can then use LibreOffice to handle the conversion between the various formats you want to support. The downside to this is that you'll eventually outgrow WordProcessingML if you want to support futures not supported by that specific format.
Also, consider placing print preview near the print command, or vise versa. Having two locations that contain the same type of functionality can be confusing.
You should consider adding word support. Support for office XML formats[1] are simple enough to hand-roll.
[1]:https://en.wikipedia.org/wiki/Microsoft_Office_XML_formats