The small print on the website may also tell you that these attachments must not be larger than a specific size (e.g. 100KB). That pretty much rules out the larger Word document exported from Pages applications older than v6.3 on Sierra or High Sierra. If the specified size is exceeded, the upload will fail.
You can export to Word .docx from Pages versions older than v6.3, and then open and save the File again as Word .docx either with the free LibreOffice, or MS Word. This process will generate a Word document smaller than 100KB.