In order to print an unsupported document through web print, you can first convert the document to a PDF using a PDF print driver. Two popular PDF writers are:
After installing the print driver, you will have an additional printer available on your system. For example, with CutePDF you can simply click File → Print as you would for a standard printer. Then select CutePDF Writer as the printer, set your desired options, and click Print.
After clicking Print, a Save As dialog will appear. Select the filename and location then click Save. CutePDF will generate a PDF document which you can then upload using Web Print.