In Stardraw Design 7 lines can be any thickness, even 0 (or 'hairline').
A line width of 0 is interpreted by printers as 'print as thinly as possible' so for high resolution printers, e.g. 1200dpi, a line of 0 width would print at 1/1200th of an inch. This will probably we too faint to see, especially if the line is a light color.
Minimum Printed Line Width, in Tools | Personalization | Settings, allows you to define the thinnest real-world measurement to which a line will be printed, either to paper or PDF. The default and recommended value is 0.02mm or .001".
If print or PDF output appears 'blocky' or 'blobby', the Minimum Printed Line Width may have too high a value; reset to the default value or one which is your preference.