If you outsource the design of your PDF files and use more than one agency, or maybe you create some of them in-house and outsource the rest, how can you be 100% certain that your brand is consistent across your content?
The PDF is not a content creation tool, it’s just a file format. This means you have a multitude of programs available to create them, like Photoshop, Illustrator, or even Word (you animal), each with a different range of design capabilities.
How are you going to keep your visual brand coherent when there are multiple people working independently on your content, probably using different programs, pushing the boundaries of your style guidelines in different ways and possibly not even in communication with each other?
Consistent branding is essential for content marketing.
How can you expect to grow a recognisable brand if your own marketing materials are having a worse identity crisis than Gollum from Lord of the Rings?