It is not possible for visitors to upload files directly to your site. All content on your site needs to be posted by you. Here are two alternatives for you to consider:
1. You can use a form application such as Wufoo, that allows your visitors to send you a message with an attachment via your website. Click here to learn how to set it up.
2. The other option is to put an email link on your site: Link to an email address. When visitors click a link their email program will open a new message which will allow adding file attachments.