Hey @Thore,
From the error message it looks like you need to take the string you are trying to pass directly in, and first wrap it inside an object.
In the object the key will be 'base64Data' something like the error example {name: 'imageName, type: 'application/pdf, sizeBytes: number, base64Data: 'string'}. You can create this object in a transformer or Javascript Query and then send the result of that into the PDF display component.
Also for reference on using PDFs check out this very useful thread here!