Flatten transparency in PDF to remove thin white lines?

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 . 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 https://unix.stackexchange.com/questions/162922/is-there-a-way-to-flatten-a-pdf-image-from-the-command-line but to no avail: * the convert method removes the white lines but converts all the text into a big raster image; * the pdf2ps | ps2pdf and gv methods do not remove the white lines (I suppose that poppler and gv have 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?