Download the pdf form that you need to complete. Realize that it isn't a fillable form. Cry.
Stop crying. Open The Gimp. (Don't have The GIMP? You can download it here. It's a free, open-source image-editing program. Like Photosop. But free.) Open the pdf form you need to complete by clicking "File" and "Open". An "import from pdf" dialogue will open. Click "Import".
Choose the text editing tool from the list of tool options. It looks like a capital A. (You can also choose "Text" from the "Tools" drop-down menu.)
Create a text box in the first section that you need to complete by clicking in one corner and dragging to the opposite corner. Then you can start to type, and whatever you type will show up in the box you just created.
When the form is complete, go to "File" and choose "Export as".