This is for all you non-German speakers out there. Yes, it is indeed possible to download a PDF of manuscripts at the ONB in Vienna!
All the fully-digitised manuscripts for the Österreichische Nationalbibliothek are listed on a page here: https://manuscripta.at/lib_digi.php?libcode=AT8500. (The link doesn’t look very permanent, so you might have to search at manuscripta.at). [Update: drill down from https://manuscripta.at/digitalisate.php]
But it is very useful to have them all on one page! Ctrl-F to find the manuscript you want by number.
Here’s a screen-grab of the top of the page:
Click on the manuscript you want. I’ve highlighted ONB 6. Here’s the next page:
What you want is the “Volldigitalisat” – “Fully digitised”. Click on the “Quicksearch” link:
I had to use Chrome’s automatic translate facility to work out which, if any of this, was relevant. It’s “Online-Zugriff”. Click there. That will take you down the page, to somewhere seemingly random:
Clicking on “Digitales Objeckt” will, at last, take you to the online manuscript.
Right-click on the image, and the menu above will appear. This contains the exciting words “Objekt herunterladen” – “Download Object”. If you click on this, you will be prompted to download a PDF of the “Gesamtes Objekt” – the whole thing.
Marvellous! Well done the ONB. Now I can mark up the PDF and do some work on the manuscript.
Update, 11 March 2023: the website has now changed, and none of this now works. There is a download link on the manuscript itself, but this does not seem to work. If anybody knows where they have hidden the link, please add a comment.