Do not know much about this, but trying few things, I found this
to produce good image in the PDF file (no white stripes on it)
---------------------------
gr = ContourPlot[x^2 + y^2, {x, -1, 1}, {y, -1, 1}]
Export["file.pdf", Rasterize[gr, ImageResolution -> 128]]
--------------------------------
You can change the options to Rasterize to see which gives
you best image in the PDF.
To obtain more information on the pdf file generated by
the export, you can list the Elements. something like:
---------------------------
lis = Import["file.pdf", "Elements"]
Grid[
Table[{lis[[i]], Import["file.pdf", lis[[i]]]}, {i, 1, Length[lis]}],
Frame -> All]
------------------------------
Another thing to try is to issue the command
SetOptions[$FrontEndSession, PrintingStyleEnvironment -> "Working"]
first before exporting. Again, this is something to try.
But the Rasterize'd image in PDF do not have the white stripes
any more.
--Nasser