To add arrows to a PDF document using pdfFiller, follow these simple steps: 1. Open the PDF document in pdfFiller. 2. Click on the 'Arrow' tool in the toolbar. 3. Click and drag on the PDF document to draw the arrow. 4. Adjust the size and position of the arrow as needed. 5. You can also customize the appearance of the arrow by changing its color, thickness, and style. 6. Once you are satisfied with the arrow, click 'Save' to apply it to the PDF document.