IMPORTANT - This forum has spam file attachments, please read!
Please note, your forum is spammed, you use old version of wpForo forum plugin, it doesn't have strong spam protection, please update to latest 1.2.0 version. You got lots of spam PDF files in /wp-content/uploads/wpforo/default_attachments/ folder. Please delete those manually through FTP or update wpForo to latest version. Once you updated navigate to Dashboard > Forums > Tools admin page and find "Possible Spam Attachments" section. It lists all suspected files and allows you delete.