Hi all,
Thank you for the question!
From my knowledge, this is a common issue with Anti-Aliasing in PDF readers. What you are seeing here is an "On Screen Display Only" thing and those line are the connections between the 2 fills of each wall skin.
What is happening is that the pixels on the screen are "Anti-Aliased" which means the pixels will try to smoothen the lines' sharpness by averaging the colors displayed on the pixels around it resulting in a smoother line. This is a technique used for displaying non-orthogonal lines and curved lines as smooth lines as opposed to displaying them as jagged and pixelated lines. With the Anti-Alaising technique, the pixels around the line will attempt to average the colors around resulting in this grey-ish line appearing.
If the Anti-Aliasing is switched off, these lines will not show, however, it depends on each reader. Please note that this is only an On Screen Display thing and, if you print the PDF out, the lines will not show.
I hope this answers your question! Should you have any further issue, feel free to ask me again!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT