I'm having the same issue. I am using Avira anti-virus, the most recent dev version of Opera on Windows 10, and I am located in Canada.

Do you use any extensions? If so, have you checked with them disabled?

I just did some testing, and it seems the Google Translator extension I was using causes problems with the PDF Viewer. When it was disabled PDFs could be refreshed over and over without an issue.

I am also using Windows 10. Sorry, I should have mentioned that. When I look at the plugins in Opera I only see the Chrome PDF viewer, nothing else, so I don't know if something could be interfering with it.

What about 41.0.2349.0?

When I was writing this last night I tried to see if there was an update, but it said it was up to date. However, I just updated it to the newest version and I am still seeing this problem. The first PDF I looked at had similar behaviour, loading the first time I looked at it, but failing to render it when I refreshed the page.

I've been have a lot of problem with viewing PDFs lately. I have the Chrome PDF viewer enabled on the plugins page. Sometimes PDFs load, but other times they do not, and I just get a grey screen. To make things even more vexing, sometimes the file will load properly, but if I go back to it later, I get a grey screen. For instance, I was trying to read this file
https://www.nsa.gov/news-features/declassified-documents/venona/dated/1940/assets/files/1mar_frag_prague.pdf
and it originally loaded correctly using the plugin. However, when I returned later I just saw a grey screen and it never loaded.

What happens if you disable hardware acceleration?

I had not considered that. It appears to work much better, with no problems with the browser not responding with hardware acceleration disabled. I am using an older laptop, so I do not have the greatest graphics card. Wondering what changed between versions to suddenly cause problems loading content.

I also installed Developer 40.0.2280.0 and encountered the same problems, while some videos load, such as those on twitter, the browser completely stops responding.

I just updated the browser and sadly saw no improvement.

I recently upgraded to the newest beta version, 39.0.2256.15, and since then I have sudden started to have video performance problems on some websites. Youtube does not seem to have any problems, but I have started encountering problems on Twitter, the BBC and a few other sites. When I start an embedded video in a Tweet my browser will lock up for a few seconds, and I will not be able to scroll or perform any actions until the video loads. Any attempts to scroll during this time seem to be performed after it unlocks and starts working again. The BBC website has similar issues, except the video often does not start and I simply hear the audio with a black background. I am wondering if anyone else has been having video issues with the newest version, or any thought on what may be causing it.