Need to convert the existing doc/xls to PDF using c#

You can try using iTextSharp.
You can download library from sourceforge.net/projects/itextsharp/

Hi, depending on where your application is hosted if possible you can just try using an office automation to do this conversion.
You can use with C# code a word automation functions to call the Document.ExportAsFixedFormat method and also with C# code an excel automation to call Workbook.ExportAsFixedFormat method.
If this is not an option for you (these interop libraries are not recommended on server side) then take a look at the following 3rd party tools that have a free version (free version has some file size limitations, but it is fully featured).
This one is a word processing component for C# and here is how you can use it to convert your Word documents into a PDF files in .NET:

string inputFilePath = "Document.doc";
string outputFilePath = "Document.pdf";
DocumentModel.Load(inputFilePath).Save(outputFilePath);

And this one is an excel processing component for C#, also here is how you would achieve the same, conversion of Excel files into a PDF format from C#:

string inputFilePath = "Spreadsheet.xls";
string outputFilePath = "Spreadsheet.pdf";
ExcelFile.Load(inputFilePath).Save(outputFilePath);