PDF generation file size limit needs to be increased (TIFF).
The current allowed maximum file size in plus for TIFFs appears to 19mb. The files that do fail this check are often between 19-21mb so a small increase would stop a lot of errors.
e.g. this file is 19.3mb (EMS85521)
Please increase the limit to 24mb (as per this slack conversation: https://europepmc.slack.com/archives/C0PFLUZU3/p1579788721030400).