How do you decide what format to publish your content as?

I do not think there is one right answer to this. 

I would recommend that you test and measure. Create content in different mediums (blog posts, infographics, PowerPoint presentations, PDFs, podcasts etc.), and see what medium drives the most engagement with your target demographic.

For example, you might find that if a person reads a PDF whitepaper by your company, then they are more likely to buy your product, than if they just read a blog post. 

Using Marketo, you can track what content people see during their customer journey, and then analyze the downstream effects. Then reallocate your budget for content creation based on what maximizes ROI. 

For freemium stuff, our newsfeeds delivered via email are doing well for lead generation and for upselling.  
For keeping our customers engaged our videos, podcasts and blog are gaining some traction.  
For our paid subscribers, they prefer the print and PDF versions of our publication because they can bring it with them when they travel or share it.  We did away with some of our PDF publications because of copyright violations and opted using flipbooks as an add-on feature to our publications.