You view your PDF on screen and — gasp! — tiny white lines! Disaster or not?
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.