I had a old standalone application which i developed in vb.net creates drawings. Now i want to develop the same application in web and when my client select the drawing options it should generate a drawing in dxf or pdf format.
In your case, you want to give user a PDF or dxf (a text file, or do you really meant for dwf?), which would be downloaded and/or be opened in browser.
So, the pdf/dwf/dxf would be generated by your web application in the web server with or without AutoCAD installed. I am not sure what kind of lisence policy is applicable of installing AutoCAD in server side. Technically, running AutoCAD in non-user-attented server environment is problematic, to say the least. Since AutoCAD 2013, you might try to run AutoCAD console (I'd only try Acad 2014 or later, console for Acad2013 is more like experimental).
Since your old app is an EXE app, which very likely uses AutoCAD COM API to automate AutoCAD session, you really cannot use the same or similar process in a web server environment.
Yes you are correct "application in the web" . Yes its dwf or pdf file needed as output. Example : in the web page my client want to select motor model, type, speed, capacity and available space etc and based on that input it should create CAD drawing in background and produce them dwf or pdf output.
Regarding server side Autocad 2014 will do as per my cad sales agent.