Writing a stream of strings into a PDF file will not make the document a PDF document. It is more applicable to a Text document. To create a specific document such as PDF or doc, the stream needs to be converted into the right format. Likewise, a message sent by a web service is not formalized into a PDF. Although the message is written into a PDF file, it cannot be opened by a PDF reader. So before being written to file, a message should be formatted to PDF format first.
A workaround is to use JavaMethod activity to formalize the message into PDF binary code and map the binary code to MIMEPart of a SendMail activity. The PDFUtil.jar (attached) is a support written JAR file that can be used to convert a String into PDF binary code and the itext-2.0.6.jar (attached) is its dependency.Find the sample project TestEmailPDF.rar attached for more details.