Change files order :
In the right part at the bottom ( PDF Editor for JIRA Panel) you can change the files order using the Up and Down arrows and choose the files to be exported using Select / Unselect action
Once you have selected and ordered your files, you can start the export using the Export Action, please see below:
Note that Export Action will be export all selected attachements on one PDF file which will be attached to the Jira issue.