Attaching files related to a Salesforce record

Hi, I’m trying to find a way to use files attached to a Salesforce record in a DocGen package. In other words, there are pdf files that appear on the Salesforce record’s related Files list, which I want to include in my package. Ideally, these would end up merged into a single pdf with data from the main record. Alternately, they could be included as separate pdf files. At this point, I’m not clear about how I would even retrieve those files into my DocGen package, or if it’s even possible to do so. I’ve already granted you access to the org: https://canadiancancersociety--devgms.sandbox.lightning.force.com/, org id = 00DAq000009Bvqb. 

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