Since you have not clarified for us whether you are using a local host or just viewing the text file in the browser I will assume the latter.
In this case, you would have your anchor tag and in the href attribute put the location of the file
<a href="C:UsersShannon MyersDocumentsBiology-DNA een_time_report.pdf">ok</a>
In the above example I started from my C: drive and went from there. Also you could do it as follows:
<a href="Resources een_time_report.pdf">ok</a>
This should link to the pdf file that you have in mind, as long as the resources folder and html file are in the same folder