I've also attempted to render a form based pdf to the filecabinet, and then set that pdf as the src attribute along with other pdfs within the file cabinet. This also gets the same internal server error.

I believe the PDFs in the File Cabinet needs to be "Available without login" to be referenced as a src. Hence why external PDFs are working for you, but not internal PDFs.

Thanks Josh. All of these pdfs are stored in the same folder of the file cabinet available without login. There's no internal vs external pdf issue here unfortunately.