Re: Compatibility with 2003

How do you use the Open XML SDK to create files that are compatible with older versions of Excel (97-2003) without the user having to install the compatibility pack? I need to be able to create .xls files and send to the browser. These files may have multiple worksheets.

OpenXml format/structure is something completely different than what excel 2003 and before used. Before there a native MS Office format was used, now everything is put into xml files that are organized in several directories, then zipped into a (in case of excel) .docx document. You can just rename the docx to .zip and extract to see this.

There's no way you can create old-style documents using the OpenXml SDK, as that's specifically developed to manipulate the xml type of documents, not the old style stuff.

I suspected as much. Can you recommend an SDK that would create the 97-2003 spreadsheets without having to install Office on the web server?

As Ondeugd said, the Open XML SDK doesn't support older (pre-openXML) formats that Office used.  I believe there are other tools that can help with those formats but we're not focussed on those sorry so you'll have to search the web for them.