HP 3840 - Manual (Page 51)

2. In the Scaling (%) box, type the percentage by which you want to shrink or
enlarge the document.
• Type a number from 25 to 99 to reduce a document image.
• Type a number from 101 to 400 to enlarge a document image.
3. Click OK, and then print the document.
HP Deskjet 3840 series printer User's Guide
51