@magicalclick: NVM, turns out there's a variety of reasons why you might not be able to open a document on a WP:
You might not be able to edit a Microsoft Office document on your phone depending on what's in it or how it was last saved. Here are some of the more common things that make a document read-only on your phone.
The file is:
Saved as an older type of Microsoft Office document, such as a Microsoft Word 97-2003 document (.doc), Microsoft Excel 97-2003 workbook (.xls), or Microsoft PowerPoint 97-2003 presentation (.ppt).
Marked as final.
Protected using rights management.
Protected with a password.
Protected with restricted editing rights.
Microsoft Word documents are read-only if they have:
Formulas that contain links or references to other workbooks.
Formulas with structured references.
Formulas that contain references, content, or formatting beyond cell IV16384.
Formulas that are longer than 512 characters or have more than 30 arguments in them.
XML markup that isn't formed correctly.
OLE objects or ActiveX controls.
For Microsoft OneNote notebooks, you can't edit:
A page from a notebook that was sent as an email attachment.
A notebook that is shared on SkyDrive, a Microsoft SharePoint site, or a SharePoint Online site, and marked as read-only.
A notebook that has a corrupted page.
For Microsoft PowerPoint presentations, you can only make certain types of edits on your phone. Some examples include editing text in a text box, adding or editing notes, changing the order of slides, or hiding slides. For more info about editing a PowerPoint presentation on your phone, see Use PowerPoint Mobile.