Hi @esthervt, Acrobat Reader always leaves a 2.5mm blank area around the edge of the paper. So if all your margins are 0 and you have a line inside this 2.5mm area, it won’t be printed. You can have a look at this topic that I did a few years ago: Tiled preview doesn't match printed pages?
You can also test using different sizes of paper, in the Layout tab, to see which will put the pattern pieces closer together. I always put a check mark next to the Save Length of Paper option when I do this.
However, looking at the image that you posted in your 1st query, I wouldn’t concern myself with the pattern touching the upper part of the image because that is just an image of how the pattern will appear on the printable parts of the paper. Those margins are only the way Seamly2D calculate if the pattern will fit onto your chosen paper or not.
When you go to the next step of actually creating the PDF file, you will have options there to create margins for the actual paper and that line will be on the margin that you set there.
I hope that this will help you