The Save Multiple Reports capability was introduced at Build 168 of 12.1 Web Viewer.
The Save Multiple Reports option only works with Text reports. From the Solution description:
Requirement to save multiple text reports to a single file for input
to another program. Selected reports need to concatenated together
("stacked") and in user determined order.
You select multiple reports from your report list, then click the Save button/icon. The "Save Multiple Reports" dialogue opens and you may reorder the reports to be saved in the "Reports to be Saved" area, selecting and promoting/demoting that selection. In the "Concatenated File Name" area, you may accept the default or change it to whatever file name you'd like. Click the OK button to execute.