Unexpected server response (301) while retrieving PDF from arxiv

I got the following message when I tried to annotate PDFs from arxiv in obsidian (with the Annotator plugin). I use "..." for brevity.
Unexpected server response (301) while retrieving PDF "https://via.hypothes.is/proxy/static/.../https://arxiv.org/pdf/...pdf".

The related markdown is
`annotation-target:: https://arxiv.org/pdf/...pdf`

Same syntax works well for other PDF links. Any idea what the problem is and how to fix it? Thanks.

Start by reproducing the request using the `annotation-target` URL shown in the issue and compare the arXiv PDF response with a PDF link that works. Trace how the proxy handles the arXiv response and its 301 status. Done means arXiv PDF retrieval succeeds for the documented annotation syntax.

Written by the indexing model from the issue text.