Very sparse contour line labels in PDF export

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Contour line labels are dynamically enabled/hidden depending on the zoom factor, which of course makes sense. The more I zoom in, the more labels appear (and vice versa). However, in my PDF export I only see very few labels, altough the output size is large and labels small. In certain areas there is no label at all. 

Why are so many labels suppressed, when exporting as PDF?
Is there a way to configure that, apart from enabling all labels anytime?

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When I export to double the size, many more contour labels appear, exactly like on screen. However, I need a way to control this explicitly.