I have a invoice.jsp in which i extracted the html part using copy tag and saved it in a string say htmlMessage.Now there is another page to send the mail sendMail.jsp which sends that string to customer.But the customer is receiving the invoice in text format.I need that it can be send as attachment in PDF format.
Thank you...

You need to use an external PDF generation library. There is currently no clear winner in this space. From a quick search on Google, though, it looks like PDFBox (http://www.pdfbox.org/) and iText (http://www.lowagie.com/iText/) are pretty popular.

Jon Emerson
http://www.jonemerson.net/