We created a report using the Visual Report Writer but in some PCs in our customer the report, when they export to PDF, the file generated comes out like this:

Click image for larger version. 

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The black marks are some lines that we put on the report like a border.

Does anyone know something about it and how to fix it? Because we didn't set up the lines like in the image. We used normal lines and we want it.
In the same customer, if they export this report using the Win7 the PDF comes out normal. But when some end-users export in their PCs using WinXP the lines are like in the image.
We can't reproduce it here in our lab.