Is there any way to put a Back button on a page so that no matter which page you just came from it will take you bact to the last page?
That's what the browser back button does already, right? Most people would regard this as a confusing duplication, I think. Or are you asking for something different? The meaning of "last page" is not clear in your post.
Actually, this is completely different from my original question, but want I would ideally like to do is be able to have someone click on an image of a document, then it opens a .pdf file, I have seem this done before, just not sure how exactly to do it.
<a href="the_pdf_file.pdf"><img title="click to open the PDF" src="some_image.jpg"></a>