patl - I missed these replies.
There may be some confusion -- the Merk Manual 20th Edition is available in digital form; it's simply not a conventional PDF. However, because it's so well organized and consistently formatted, it translates to PDF very well.
If you would like a copy of the digital edition, it's available free on Android and Apple IOS (Android version below):
[Login to see the link]
Although this is not the PDF, in many ways this is better because it has an improved search and integrated videos. Again, it's also completely free.
If you do not have an Android or IOS device, don't worry. You can use any number of Android Emulators (Virtual Machine) that allow you to use the manual on your PC.
In my opinion, the easiest, more feature-rich Android VM is Memu. This is a point-and-click install. It requires no technical knowledge whatsoever and the emulator allows cut-and-paste (many do not) so that you can use it much like a native Windows app. It's available for free: [Login to see the link]
I hope this helps, good luck.