You need to read the PDF (filereader), then you can convert that File-object into a byte Array. That is pretty simple, because any object can be converted int a byte array.
Reading, Writing, and Creating Files @ Java Tutorials[
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public static byte[] getBytes(Object obj) throws java.io.IOException{
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos);
oos.writeObject(obj);
oos.flush();
oos.close();
bos.close();
byte [] data = bos.toByteArray();
return data;
}