What is the difference between "must be" + verb + ing, and "must have been" + verb + ing?
She must be looking for this document, means she is still looking. You politely ask if it is what she wants; unless your intention is for her not to have it.
She must have been looking for this document, means she has stopped. The document in question was likely overlooked since it is still there. Maybe now is the time you should ask her if this was what she had been looking for.