This one has consumed my entire weekend, and still we can't make mailing labels. Running Civi 4.7.15 on WordPress 4.7.2 ... successfully installed and tested wkhtmltopdf only to discover Civi doesn't use this app for mailing labels. Paths in TCPDF config appear to be good, and I haven't modified the code except for the config file (which I did after discovering the malfunction). Debugging shows no errors, yet every attempt to make mailing labels displays "TCPDF ERROR: Some data has already been output, can't send PDF file."
What might cause this?