Under Windows you can use WinWget portable to update all RME manuals of interest quickly on demand.
Especially useful, as some of the manuals got many updates last recently.
Additional benefit: its easier to search for keywords, when having the PDF files stored locally on disk.

1. get this software: http://portableapps.com/apps/internet/winwget_portable

2. create a file containing the Handbook URLs, i.e.: downloads.txt
https://archiv.rme-audio.de/download/adi2pro_d.pdf
https://archiv.rme-audio.de/download/ff … plus_d.pdf
https://archiv.rme-audio.de/download/octamicxtc_d.pdf
https://archiv.rme-audio.de/download/raydat_d.pdf
https://archiv.rme-audio.de/download/adi2pro_e.pdf
https://archiv.rme-audio.de/download/ff … plus_e.pdf
https://archiv.rme-audio.de/download/octamicxtc_e.pdf
https://archiv.rme-audio.de/download/raydat_e.pdf

3. start WinWGet and create a job, i.e. "RME handbooks", where you
    - define the location of the input file, here downloads.txt, which contains a list of URLs, one per line *)
    - define the location on disk, where the manuals shall be stored *)
    - enable timestamping options
    See screenshots below ...

4. start the job ...
    *) in case of an error, try to use directory names without blanks (" ").

Caveats: needs further testing whether download will happen after a handbook update. I hope that I got the options right.

Screenshots: