diff --git a/lib/document.coffee b/lib/document.coffee
index 6b97659..6137e54 100644
--- a/lib/document.coffee
+++ b/lib/document.coffee
@@ -98,11 +98,11 @@ class PDFDocument
                @generateXRef out
                @generateTrailer out
                
-               ret = ''
+               ret = []
                for k in out
-                   ret += k + '\n'
+                   ret.push(k + '\n')
                    
-               fn ret
+               fn new Buffer(ret.join(''),'binary')
         
     finalize: (fn) ->
         # convert strings in the info dictionary to literals
@@ -168,4 +168,4 @@ class PDFDocument
     toString: ->
         "[object PDFDocument]"
         
-module.exports = PDFDocument
\ No newline at end of file
+module.exports = PDFDocument