DEVONthink ToGo takes “pictures”. They don’t (yet?) provide a feature to scan a document.
I avoid this problem by taking a scan and exporting that into DEVONthink ToGo (if that’s where I want it to go). While I normally use (for years) the app “Scanner Pro” there are other apps, including Notes on iOS that scan in B&W, grey scale, and color which might work better for you. Certainly the file sizes are smaller and I would say the results are better than using a photograph.
Suggest you try Notes first. Already on your iOS device. Scanner Pro has some great features (creates PDFs, for example) and I’ve used for years, but now has competition. I use “share” button to get into DEVONthink. Not free, though. Also Dropbox App has scanning. Probably others.