You raise a good point. You are not the only person to request the ability to add documents to your family site, there are other members who are requesting the same capability. We certainly take your suggestions into consideration and will do our best to add the function as soon as we can.
Currently, you can add photos and video to your family site in the Albums section.
If you’d like to add documents, the best thing to do would be to create a News article and copy the text from your document there. The advantage of this is that the News article will be displayed on the front page of your site so everyone that comes to visit your site will see it right away. You can also attach photos to the article if you wish.I hope this advice is useful to you.