Zoom refers to what you do, when you look at an object and then press some button to make it larger on your screen or viewfinder. For that reason, it is called a "zoom lens."
To enlarge, is to make the resultant OUTPUT (usually a print copy) larger than the original.
So therefore, you will want to "enlarge" the copy, not "zoom" it.
Now, I am troubled by "enlarge the word in bold for one times." I wonder if you mean, "I want to enlarge it by 100%" which would take a 2-inch word and render a resultant copy that was 4 inches? Even that is questionable, as some might say that 100% larger is the same as the original. So maybe you might want to say, "enlarge it to twice the orignal."
I, too, am often bewildered by this silly language!
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