Sumatra PDF is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows

Moving tabs about

Moveable tabs, like in Firefox, would be very helpful to keep related pdfs grouped together. But only if Sumatra would remain the light and fast program it currently is.

Tabs do move, just drag to right or left

The ability to move between windows is a long time open request https://github.com/sumatrapdfreader/sumatrapdf/issues/18

Combining groups of files into one tab is possible using in pre-release myfiles.vbkm feature , however it is still fairly experimental (page numbers are not what may be expected)
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Blimey, I haven’t been paying attention. Sorry & thanks!