The PDF format limit’s our ability to fix and check your files.
The files are prone to several major issues that can impact your final product.
These include:
Fonts and images not properly embedding
Improper flattening of transparencies which may result in boxes or lines forming around images and text.
Elements set to “overprint”, which may drop out when printed. 

You can choose to continue with PDF submission, but we cannot be held responsible for any missing or incorrect elements once proofs are approved due to the inconsistent nature of the file format.