If a PDF is high-resolution, then it can take a long time to rasterise it (convert it into pixels) when creating thumbnails. The browser can therefore time out before this process has finished. One solution to this is to use bulk upload, which will perform the asset import asynchronously so the browser won't time out.
It is also possible to change DPI value using this setting:
The default value is 300 DPI, but if the time outs are occurring regularly this value can be reduced, for example to 72, which will significantly reduce the time it takes Asset Bank to create thumbnails. Note that if the quality of the thumbnails created for PDFs is not good enough then you have probably lowered this number too much.