OK. Having realised the problem was to do with the type of file (pdf) rather than the file size I was able to find a previous thread which deals with exactly this problem.

Upload PDF file to Documents in Asset Library

In summary though, the problem is caused by the MIME type which is embedded in the file by the web browser. It seems there's a problem with Firefox, which is what I was using. I tried uploading a pdf with Microsoft Edge and it worked fine.