In Map Viewer, on the Map only tab of the Print panel there is a field that allows you to specify the file name of the exported map document:

In the help page it says: 'Type a file name for the map document.'
When exported, the Exports window shows that the file name of the exported map document to be the value entered in the relevant field:

However, opening or downloading this file shows the filename of the exported map document is something else completely. In the above example, the filename generated was:
x_____x4bXdzBU2568MD6PXoWXXGw..x_____x_ags_b2131163-92d5-11ef-b550-12fb48fcd9e1.pdf
This seems like a bug but I can't find any record of it anywhere. Just wondering if this is a known issue or if the 'file name' field isn't meant to be used as the name for the file?
Tested in both Chrome (latest version) and FireFox (latest version) and it's the same behaviour in both.