All thumbnail images of File:Mueller_letter_to_Barr_2019-03-27.pdf are served with HTTP 500 or 429 Too Many Requests. The file itself is a PDF document, version 1.3 without obvious anomalies.
Specifically, all links //upload.wikimedia.org/wikipedia/commons/thumb/4/47/Mueller_letter_to_Barr_2019-03-27.pdf/pageₖ-ₙₙₙpx-Mueller_letter_to_Barr_2019-03-27.pdf.¤¤g fail for whichever ₖ = 1,2, various ₙₙₙ, and ¤¤g (format) either png or jpg. Due to error hiding, not much information is available for me.
Such links are expected to serve thumbnails in image/png or image/jpeg depending on the extension.