I had the problem of trying to open some .QDF files.
A google revealed they were most likely Quicken files, but had little luck going down that path.
Eventually I found out they were Reckon files. The QDF extension was a bit confusing, until I read this. In Australia, Quicken was localised by another company who it seems have parted ways. This resulted in Reckon being the Australian ‘version’ of Quicken.
Intuit owns Quicken, as well as QuickBooks. They have an online version too, descriptively called QuickBooks Online.
The result of all this was that I had Reckon files, with a .QDF extension – but an export of the files were also provided, and they had a .RKN file extension instead.
If you want to view these files, Reckon provides 60 day trial software that seems to have no limitations, available here. Note that if one version prompts saying you need to purchase addins, try a different version instead. I had better success with Reckon Accounts Home & Business 2016 with the particular files I was working on.
Also, if you get stuck then you could try some of Inuit’s free conversion utilities from older Quicken files to newer Quicken files, which can also be read by Quickbooks.
Credit to Reckon’s support who explained some of this to me!
I hope this helps anyone trying to open .QDF files or .RKN files, particularly if you’re from Australia!