When You See Thin White Lines in Your PDF Files
Have you ever opened a PDF file and seen thin white lines where there shouldn't be any? In general, the white lines, or "light leaks" are due to a PDF that includes flattened transparency -- transparency effects (such as placed PSD files) in a file saved in a file format that doesn't support transparency (such as Acrobat 4, a.k.a. PDF version 1.3).
Flattening transparency means "faking it" by creating opaque areas that look transparent. But these opaque areas have to fit together like a mosaic, each piece right next to the next. The result is that sometimes white peeks out between them. In the vast majority of cases, this appears only on screen! Sometimes it shows up on low-resolution printers, too, but virtually never in high-res commercial output.
See the thin white lines around the trapazoid at the top and around the text at the bottom?
The best trick I ever learned for dealing with these came from Michael Stoddart, who said: Zoom in and out! If the white lines are always one-pixel thick (they don't get thicker or thinner), then they are just screen artifacts and you can probably ignore them. If they do get thicker when you zoom in, then they're really there.
This used to be a huge problem, but it's gotten more rare as time has gone by. I think the reason it's rare to see this is that Acrobat has gotten smarter. In fact, I can't even force Acrobat 9 Pro to show me those lines in the image above. I opened the PDF in Mac OS X Preview. There's another good trick: Never assume Preview (or any other pdf reader) is giving you an acurate, um, preview.
If you do see white lines in Acrobat, check the Page Display preferences, and make sure "smoothing" is turned on:
There have been other issues with white lines, such as this old CS3 problem with PSD files (which I haven't seen in a long while) and this even older bug in CS3. But again, those don't show up much anymore.