Is it cause for concern if you can see random white lines in a PDF file? Usually the answer is “no”, because these artefacts only appear as a result of rendering a print-specific file for viewing on a pixel-based screen. There aren’t actually any white lines in the PDF, and the file will not print out with these artefacts.

You may have noticed that sometimes when you view a PDF on your screen, that there can be very fine white lines and boxes throughout the document that aren’t part of the design.

Image of PDF showing white transparency lines
Closeup of PDF screenshot showing white transparency lines

Obviously this can be worrying if you’ve never come across them before. However these lines are not usually indicative of a problem. They occur when you view a PDF onscreen that has been produced to conform to an older PDF standard (prior to version 1.4). 

Over the years the PDF specification has been updated several times, and with each update has gained new features. One of these features was support for transparency. This allowed a PDF to hold layered, overlapping images and text, with support for alpha masking. Nowadays we take such things for granted, and PDFs created to this spec typically render perfectly onscreen. But before this support was added, a PDF file was basically just a wrapper for a postscript file (which is literally how the format began it’s life). Postscript is the language of printing, and as such, everything rendered by postscript must be able to be etched onto a printing plate. Postscript is all about creating a physical object and so, of course, transparency and layering have nothing to do with it. A PDF output to an older ‘flattened’ format has had all hidden layered data—information that you don’t need for printing—removed, and all transparent objects have been turned into flat graphics which are merged with whatever was visible behind. There are no layers anymore, just many different pieces of raster and vector artwork, all on a single layer.

…it’s the seams between the edges of these pieces that render out as fine white lines only when the PDF is viewed onscreen

All these different pieces are assembled like a jigsaw puzzle, and it’s the seams between the edges of these pieces that render out as fine white lines only when the PDF is viewed onscreen. Truth be told, these edges aren’t white at all: they’re transparent. They show up onscreen because the edge isn’t sitting perfectly aligned with the pixel grid of the display (this is also why the white lines change at different zoom levels). It’s impossible to show half a pixel onscreen, so the PDF viewer renders small gaps when there are sub-pixel issues.

So, is it cause for concern? Usually the answer is “no”, because these artefacts are caused by the process of rendering a print-specific file on screen. There aren’t actually any white lines in the PDF. When outputting the file to printing plates, there is no pixel grid, and no sub-pixel issues. There aren’t any pixels at all.

White lines can actually be good

It’s actually not a bad idea, if outputting a file for printing on a commercial press, to intentionally conform your PDF to an older version. Remember how everything that’s being printed has to be flattened at some point? When would you rather this flattening occur: as you output the file yourself and can check it for errors right there and then, or when the pre-press operator at your printer processes your file, outputs it as a proof, and you have no way of checking it until you receive the proofs? 

Older spec PDFs are free; proofs aren’t. Learn to love the white lines.