Ensure you are downloading the proof to your computer and open in Acrobat Reader. See: How to view a proof.
During the proofing process, your artwork might be flattened to avoid any press issues. Sometimes the way the artwork is flattened, the PDF file is sliced in to smaller pieces – this isn't a bad thing, but sometimes when viewing the PDF the gaps between these slices show (even though there is no white space between the slices).
These are usually caused by:
- Large images
- Pantone colours
The way to check if these are preview errors or an actual print problem – zoom in to the closest possible, if the lines disappear, then they are just preview errors and will not print, you may proceed to: What to check on my proof.
If the fine lines are still visible, then they may print, so do not approve your proof. See: There is a problem with my proof.