I have just created a downloadable document in a .doc format but would like to know whether you think backlinks pointing towards this file will be of any use in my site's overall trust with the search engines or whether it does no good at all?
Yes, they do. If you do search using the yahoo linkdomain:yoursite.com query or even look in google backlinks you'll probably see a variety of different source documents like pdf, doc, and .txt etc. For example, many .edu links are from pdf, doc and ppt's. Some even consider them to be given more weight.