To put the file in the iframe you'll need to see the iframe's src property on the client. When someone changes the drop down update the src of the iframe via javascript to something like "/showpdf.aspx?id=???" where ??? is the value of the selected item. Now create a page called showpdf.aspx and in its page_load event put the code that is currently in your _click event only rather than getting a value from the drop down, get it from the querystring
SqlCommand cmd = new SqlCommand("select * from employeep where Salary ='" + Request.QueryString["id"] + "'", con);
In terms of your query you shouldn't use "select *", always name your columns, and don't make SQL that way as it is open to sql injection attacks, used parameterised queries instead.
To answer your next question, not I won't write all the code for you. If you don't know how to run javascript when the dropdown changes then google "javascript select change event". To read the selected value google "javascript get select value on change". To update the iframe's src google "javascript update iframe src". Then you just have to put everything together.