Windows OS
Download & Install Adobe Acrobat Reader (can get the free edition).
    Open the .dpf document you need to sign with Adobe Acrobat Reader.
    Click on the "Fill & Sign" tool in the right-hand pane, or in the top toolbar.
    Click the "Sign" button in the toolbar that appears, then select "Add Signature".
    A window will pop up with three options for creating your signature:
        Type: Type your name and choose from several cursive font styles.
        Draw: Use your mouse, trackpad, or a touchscreen pen to draw your signature.
        Image: Upload an image of your handwritten signature (sign your name in black ink on a blank white paper first, then scan or photograph it).
    Click "Apply" (or "Done") once you've created your signature. It will be saved for future use.
    Position the signature where you want it in the document by clicking the desired spot, then drag the corners to resize it as needed.
    Go to File > Save to save the signed PDF document. 

Mac users can use the built-in Preview app to sign documents without installing extra software. 

    Open the PDF document in the Preview app.
    Click the "Show Markup Toolbar" button (it looks like a toolbox or a pen tip).
    Click the "Sign" button (which looks like a signature) in the Markup toolbar.
    Select "Create Signature" and choose a method:
        Trackpad: Sign your name on the trackpad using your finger.
        Camera: Sign on a white piece of paper and hold it up to your Mac's camera.
        iPhone or iPad: Use your connected Apple device to sign with your finger or Apple Pencil.
    Click the new signature you created from the menu to select it.
    Drag the signature to the desired location in the PDF and resize it using the blue handles.
    Choose File > Save to apply the signature and save the file.