This article has been written with the steps for Acrobat Reader DC.
First, open your .pdf in Adobe Acrobat Reader. You cannot digitally sign a document from within a web browser such as Mozilla Firefox or Google Chrome. You can ensure that you have your document open in Adobe Acrobat Reader by double clicking the file on your computer.
Fill in your form by clicking the fields and typing, as normal.
You can identify a signature field by the little 'red flag' in the upper left, as pictured.
When you click into the field you will get the following dialog box.
Click on the 'Configure New Digital ID' button. It may read as 'New Digital ID' if it is your first one.
Select 'Create a new Digital ID' then continue.
Select 'Save to File' then continue.
Enter your details and ensure that key algorithm is set to 2048-bit RSA. Typically you will use it for digital signatures, but if you also require this signature to encrypt a file (Advanced users only) or do both, then select the appropriate option in the 'Use Digital ID for' field.
Select the location that you will save your signature to. If you intend to use this signature on other computers and don't want to create a new one, saving it to a USB or portable device will allow you to import the signature (The first step of the creation process has an import option).
If you intend to only be signing documents on this computer, the default location is adequate. Set a password for your digital signature and confirm, then save.
You're done! Select your signature from the list and press 'continue'. You will be looking at your signature now, you can change the appearance by clicking the drop down near the top, and modify it if you want to by clicking the 'create' button. If this is the final signature required for the document you can then lock it to prevent further changes to the document (Do NOT do this if further office work or more signatures are required on this document), enter your password to make the 'sign' button active, then press sign.