Start by clicking on the form block within the page editor. A number of form elements should be visible on the left hand side.
Next, click and drag over the file upload form element and place it within your form.
Click on the file upload element within your form and multiple options should appear on the left.
Here you can add a label to your file upload element, allow multiple files to be upload instead of just one and make the field required so people have to add a file or files before they are able to submit the form. Individuals filling out your form can upload PDF's and other small files (25MB or less).
Here is what the file upload form element will look like on your page!