--- a/docutil/docpdf.py	Tue Aug 15 16:48:49 2017 +0300
+++ b/docutil/docpdf.py	Tue Aug 15 17:01:51 2017 +0300
@@ -48,14 +48,14 @@
 
 
 def open_win_pdf(readerexepath, pdffile, pagenum = None):
-    if pagenum != None:
+    if pagenum is not None:
         os.spawnl(os.P_DETACH, readerexepath, "AcroRd32.exe", "/A", "page=%d=OpenActions" % pagenum, '"%s"' % pdffile)
     else:
         os.spawnl(os.P_DETACH, readerexepath, "AcroRd32.exe", '"%s"' % pdffile)
 
 
 def open_lin_pdf(readerexepath, pdffile, pagenum = None):
-    if pagenum == None:
+    if pagenum is None:
         os.system("%s -remote DS301 %s &" % (readerexepath, pdffile))
     else:
         print "Open pdf %s at page %d" % (pdffile, pagenum)