Once your document is open, go to the Tools Pane on the right and select the Fill & Sign button. If you do not see the Tools pane go to the View Menu > Show/Hide > Tools Pane to select. If you do not see the Fill & Sign button select the More Tools button on the Tools pane (See below)
Then select Fill & Sign button.
When you click on the Fill & Sign button, the Fill & Sign bar will appear above your document. See the below screenshot.
From here, select the "Sign yourself" button ...
Then select "Add Signature ...
The following window will open.
You will have three options for digitally signing your document.
The Type option allows you to type your name and then select a script font. Click above the blue line and start typing your name. You can change the font using the Change style menu as seen below.
The Draw option allows you to use your mouse or drawing tablet to sign your name. You can save your signature for future documents by clicking the "Save signature" box.
The Image option allows you to upload a photo of your signature. Sign a physical white sheet of paper (for example, copy paper) and take a photo with you mobile device. Below is an example of a signature captured in poor lighting.
You will likely need to bring the image file into a photo editing application such as Photoshop or Pixlr (See similar tutorial here) to convert the background to white. Your photo should match the example below.
Once the image is ready, click on "Select Image" and then upload your signature photo.
Once the photo is uploaded select "Apply" button.
After you finished your form, save it by going to File > Save As and choose your location and file name. You are now done filling and saving your form, you can now send your PDF form off to the requester.