The PDF format is commonly used to provide electronic forms and other documents, and often these will require you to fill them out and submit them for processing somewhere. Often, such PDFs are given form properties where you can click various fields to edit them and input the necessary information; however, often this is not the case, and those distributing the form simply have it as a basic and apparently uneditable PDF.
In these cases, you may find yourself printing the PDF, filling it out by hand, and then scanning it back into your system to submit; however, this is not always necessary to do. A number of programs including Apple's default PDF handler "Preview," offer the ability to mark up a PDF document, and you can use this as the means to fill out an otherwise static PDF form.
With the PDF open in Preview, simply enable the edit toolbar by pressing Shift-Command-A, or by clicking the Edit Toolbar button in the standard Preview toolbar. Now you can click the text tool, and then click or click-and-drag on the PDF document to create a new text box. You can then add your desired text, set the font size, color, and font face, and then resize the box accordingly to position it in the form as you desire.
Repeat these steps for all areas where you would like to fill the form, and then you can even sign the form using Apple's Signature feature to capture your signature and apply it to any PDF you have open.
When the form is complete, you can save it and email it, or use Apple's Print to PDF options to send it directly to an email without having to manage an intermediary file in the Finder.