I had the same problem and finally figured it out after several hours. Was a "duh" moment. 

On the Dell Printer Hub app that loads onto your computer, when at the Default Scan box/page, do your scan settings at the top, then start your scan.

THEN (you would think you could do this first, but it just resets if you do) the last setting option is "Save Settings". Click the down arrow to open that.

There you can select file type (pdf), File Name, and just beneath the file name entry field, is a box: Save each document as individual file. UNCHECK THAT BOX!

Proceed with selecting your save location(s)...and save. It worked!

I was on hold with India waiting on support when I realized that little box existed.

Good luck!

That box no longer exists in the latest Dell Printer Hub software.  I had to uninstall and reinstall because my scanner was not being detected after another computer update.  The latest software from Dell doesn't have the option to save multiple flatbed scanned pages to a single file. HELP!

Did you resolve this issue? I'm having the same problem. Used to be straightforward but something has changed recently.

Unfortunately it seems Dell didn't add this feature in the Dell Document Hub even though it used to exist in the older Dell Printer Hub.

The workaround I found is that you can scan multiple pages into a single file from the Printer itself. 

1. Choose the Scan to USB option on the printer (and insert a USB drive into the front).  Be sure to check the scan settings, the default is for the lowest quality.
2. You can then scan from the flat bed and choose to scan additional pages until you've scanned them all.
3. Then wait for it to save to the USB (takes a minute). 
4. Finally copy PDF from USB to PO

Kind of annoying, but at least it works.

I had the exact same issue with my E514dw model. I found the only way to do it was to uninstall the latest Dell Hub version and reinstall the earlier one which restored the 'scan another document' facility.

I found my link to the old software here.

http://driver-laptopdell.blogspot.com/2019/03/dell-e515dw-driver-downloads.html

I went to the url you stated for the old software for the Dell Hub.

There are several download links and I have no idea which one to choose so that I can restore the "scan another document" option when using the flatbed.

Please help if you possibly can. Thanks in advance!

Select the scanned files you want to save into one file. Click Tool -> Merge All Files into a Single PDF. Set the file name and the folder, and click Save.