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You can only work on the top document.

So, If I understand, you want to draw a straight line on a piece that is too big to see all at once and be acurate, as seen through a reference pic right ?

 OK, if that`s the case, enlarge the reference (all are in the same document, yes ?) til you an see what you are doing. On a new Layer, place the points you want to draw from and to. As in, put a dot on this new Layer with the paint brushat both ends of the desired line.

Make the "big" Layer active, Zoom in as far as necessary on the first point. Click it with your drawing tool ( brush, pencil...) navigate over to the other point, hold SHIFT and click the other point.

TA-DA !

If I misunderstood, ignore this post and move on.