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You don't need the loop, a more efficient way of doing it is:
(vla-open (vla-get-documents (vlax-get-acad-object)) "c:/temp/test.dwg" :vlax-true)
(setq docs (vla-get-documents (vlax-get-acad-object)))
(vla-activate (vla-item docs (1- (vla-get-count docs))))