I see...
It appears that
xmlui.content_disposition_threshold needs to be a valid integer...which means it'd have a maximum value of 2147483647
(roughly 2GB).
So, you could try that value, but it won't work for PDFs >2GB obviously....those will
still be downloaded instead.
Looking at the code, you also could try setting it to "0" (instead of "-1"). It appears zero may be another way of trying to disable it. But, I'm not entirely sure if that'd work.
If neither of these options work, then I think your best bet may be to look towards an upgrade to DSpace 7 in the future (6.x will be EOL as of July 2023 & may not have any additional releases after 6.4). DSpace 7 will attempt to serve
everything in "inline" mode as long as the file format is included in the "Accept" header (which should include all the formats that your browser understands).
Tim