Sumatra PDF is by far the fastest PDF viewer and when installing it you can assign it (installer option) as the system PDF-preview provider in windows for instant pdf previews.
The preview works fine in explorer but in opus the preview-pane shows as gray when previewing multi page files (causing the scrollbar to be shown. Simple A4 pdf-files seem to work fine but all multi page documents do not. The image is updated when the pdf-preview pane is scrolled. It is also visible after changing viewer pane preferences causing a redraw/update.
This is the single thing holding me back from adopting opus as my primary file browser.
System Specs
System: Win 10 x64 Version 10.0.15063 Build 15063
Opus 12,6 x64
Sumatra PDF 3.1.2 x64
Using 4k display with 200% scaling in Windows,
HOWTO REPRODUCE
Install sumatra PDF as the system default pdf-previewer - open installer, choose options and check the preview provider option
Open directory opus, go to a folder with pdf-files, open preview-pane, click on pdf-files with single and multiple pages
The way preview handlers work, Opus simply looks up which component to use via the registry (and via the ActiveX+Preview+Office+Web plugin's configuration, which can override the registry if you wish it to), and Opus then tells that component to view the file and passes it the file details.
If the component was not working at all then there might be an issue on the Opus side (or on the component's side; we'd have to look into it to work out which).
But if the component works with some files (single page PDFs) and not others (multi-page PDFs) then I can't think of a reason that Opus would be involved. Opus is just telling the component the file's name and where to draw the window, and the rest is up to the component.
It's possible something Opus does differently is confusing the component (Sumatra in this case), but if that is the case it is hard for us to guess what it's expecting and tripping up on. It's also possible that the component (Sumatra) is just going wrong on its own.
So there may be something we could change on our side, or it may be something Sumatra needs to change on their side, but Sumatra are probably the people you need to report this to so they can do the initial investigation, since things are working with single-page PDFs and as far as Opus is concerned there is no difference between on PDF and another when it passes them to a 3rd party viewer.