Foxit PDF Reader allows you to annotate otherwise unmodifiable PDF files using its suite of drawing tools. These drawing tools feature a variety of shapes, such as arrows, lines, rectangles, ovals, polygons and clouds, but it also offers a freehand Pencil Tool that allows you to draw or write whatever you desire. If you mess up, don't worry; Foxit PDF Reader also offers a Rubber Tool which allows you to erase marks made by the Pencil tool.
Open your PDF in Foxit PDF Editor.
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Navigate to the page you wish to annotate.
Click "Comments" in the top menu, point to "Drawing" and select the tool you desire. For freehand drawing, select "Pencil."
Click and drag your mouse to create your shape. Some shape tools, such as the Cloud, Rectangle and Polygon Tools add sides each time you click on the PDF. For these tools, click a start location and then click each corner location. Double-click the last corner to complete the shape.