One of the most common questions we hear in the PrePress Department concerns those pesky “white lines” that are visible on screen in pdf files. The bottom line on the lines is:
- they are a display-only apparition – called “stitching”, they will not print
- you can make adjustments to your display in Acrobat to remove them
The Long Answer: Complex Issues of Transparency
Transparency has been available in page layout programs for quite some time now. The problems arise when you realize that PostScript – the language that is used by imaging devices for offset printing and many desktop printers – does not understand transparency AT ALL. At some point in the process from desktop to plate, transparency must be “flattened”. This can be a very complex, though behind-the-scenes, process whereby all your content containing transparency that was created in PhotoShop, Illustrator, and a page layout program is transformed into “PostScript-legal” content that can be read and correctly imaged by a RIP, yet still “looks” transparent. You may not know any of this is occurring, but it must happen in order to print.
The PDF solution
If your file is not for print (and you are not concerned about file size), there is no reason to ever flatten the transparency – the white lines will not plague you. PDF files since version 5.0 can contain “live” transparency – as Adobe realized pdf files can be optimized for uses other than printing. If your files are destined for print, however, you need to create PDF/x-1a files. These are optimized for Acrobat 4.0, and for PostScript output. Now, this is where the dreaded “white lines” issue arises. The process of flattening to create this file “cuts up” your page content into pieces or atomic regions, and these appear to have tiny white gaps between them. THERE IS NO GAP! The “pieces” fit perfectly together. (In fact, you could probably never print any lines that fine on an offset press anyway due to dot gain.)
The Visual Fix
Now – if print is not your issue, but you simply want to view the pdf WITHOUT the white lines showing, go to your Acrobat Preferences, and in the Page Display Tab UNCLICK “Smooth Line Art” and “Smooth Images”. By doing this, you will create another problem — if you had turned any of your text to outlines, that will now display poorly and pixellated. Again, it will NOT print that way. These are just unfortunate side effects of bridging the gap between software that handles transparency and PostScript imaging language that does not.
You will find plenty of insight in the Adobe forums from users just like yourself who were left wondering “How do I get rid of these stupid white lines?!”