Thanks for pointing that out. I’m not sure what happened with the DOI — I’m already in contact with ResearchGate to get that fixed. But the PDF is still available and can be viewed or downloaded directly from the ResearchGate page, just below the summary.

As for arXiv: I haven't submitted it yet because I need an endorsement and I'm not advanced enough to get one. Because I'm new and haven't written much, I haven't been able to get one yet. I hope I will down the road — I'd really like to submit it there when it's done.

I'm able to open the PDF.