Blank PDF outputs for large report - but not if run as background activity.

[Greg C]

-- IBM Cognos Analytics 11.0.13.1

Hey all, this has got me thinking I have an environment setting I need to change or I've run into a bug.

We have a report that can output a potentially large number of pdf pages, up to about 1000.

If users run the report directly as PDF, and there is more than about ~250 pages, in the result viewer the first page appears normal, but the remaining pdf pages are absent from the viewer. 
What's more, if they attempt to save the file, they are offered only to save a file called disp.htm which contains no pages of any kind.

HTML behaves normally, as does excel.  If I run the task in background and download the result I get the appropriate output.

In testing some other large output to pdf reports, it does seem that the problem is prevalent in other reports.

We did upgrade from cognos 10 last year.