Folks, I think there’s two different things going on here.
There is the stitching issue. You can find info about that online. Here’s one page talking about it.
If you render the PDF into Photoshop, and the white lines disappear, it’s a stitching issue. Another test is to zoom in on the white lines. If they stay the same size as you zoom in more and more, it’s a stitching issue.
Every once and a while, I’ll render an Illustrator file into Photoshop (usually because the client has requested a high res JPEG file), and a thing white line will show up. Look at the attached screen captures. This is a logo that was created in Illustrator. It’s a solid black color – no spot color, not gradient, no transparency – just straight black. I rendered this into Photoshop and then added solid colors on a layer underneath the logo. You’ll see either white or yellow showing through a misrendering (for lack of a better word).
Further complicating the situation is that I can render the same file twice and one time it will be fine and the next time I’ll get a line.
To my eye, what Rebekah posted looks like the rendering issue I get sometimes – sort of like those white lines are semi-transparent – not pure white.
I wish I had an answer as it’s an issue for me.