What do you need? A binary stream (something that is derived from java.io.InputStream)?
In that case, you wrap the result from pdfUtil.generatePDF() into an instance of java.io.ByteArrayInputStream.
Pega usually stores binary data in Strings, and as binary data may contain binary values that are not allowed inside a String, you cannot use new String() with the result from pdfUtil.generatePDF() as argument; instead you need to convert that output into BASE64 first; for that conversion you have several options. The outcome is now a Java String that can be put to text properties on the Clipboard.
So please check what you really needs; what type the target property has, or what type is requested on the signature of the function you want to use.