To underline in PDF documents, upload your PDF documents to pdfFiller, or choose the PDF you'd like to annotate in “My Forms” page and click “Fill” button on the right. You can find the “Line” button in “Tools” tab. Select the area or sentence you'd like to underline by clicking and holding the button on the mouse to draw a line around. Release the mouse when you're done. You can change the size of the line using the editor toolbar.