Huh... Google Docs, maybe?
You're a bit vague on the requirements here, but by "document" I assume you mean text documents (.doc(x), .rtf, .txt), presentations or the occasional PDF file. In which case Google Docs does all of that well, and allows you to create files in its own format or upload them in their normal format.
It's not entirely HTML5 (debatable buzzword, really), but it uses some.
EDIT: following your clarification in a comment to your question, you could use Google Docs for that, it has an embedded viewer that you can put insert in web-pages. You'd need to have the files uploaded to a Google Docs account and then view them with that.
- Embedding Spreadsheets
- Embedding Presentations
- Embedding a PDF File (assuming it would work similarly for documents)
EDIT2: Actually, you can (or at least used to be able to) embed files that are not even hosted on Google Docs, read this 2009 post on the Embeddable Google Document Viewer.
