I have a PDF created with Quark Xpress where some raster images are shown with thin white lines across them. Searching around I have found that the problem is due to the way PDFs readers apply partial transparencies, see http://indesignsecrets.com/when-you-see-thin-white-lines-in-your-pdf-files.php.
The suggested solution is to either 1) turning off the "Smooth images" setting in Acrobat Reader or 2) ignore the problem because it will print just fine. Solution 1 requires efforts from the readers and is thus not practicable. Solution 2 is also not applicable to my case because these PDFs will be mostly (or only) be seen on monitors and not printed.
I tried to flatten the images using the answers given to question Is there a way to flatten a .pdf image from the command line? but to no avail:
convertmethod removes the white lines but converts all the text into a big raster image;
pdf2ps | ps2pdfand
gvmethods do not remove the white lines (I suppose that
gvhave a switch equivalent to the "Smooth images" setting of Acrobat Reader, but I could not find it).
How can I post-process the PDFs so that the parts with raster images are flattened and no white lines appear any more?