I have a data.table set up which works and gives the expected search results, however when I added the output buttons for PDF, Excel, Copy, CSV not all of the results are captured but only the resulting searches which make up one screen only. Is this intended or a bug in which case I will report it.
We'd need more information to properly help you.
Are you using server-side processing for the datatable? If you are, you'll only be able to export the visible data. See documentation of datatables