Hi,
you can use <a><img /></a> and set the href property to a .aspx page dedicated to display pdf or docs in general. Use query string to pass the file name or other parameter to the dedicated page and in the Load event of this page, run your code that redirects to the file.
Example: <a href='DisplayPDF.aspx?filename=file1'><img /></a>
Page_Load(object sender, eventargs e)
{
string fileName = Request.QueryString["filename"].ToString();
...
}