This is more designed for a self hosted server, so if you want multi-device web access then it's a great solution. I can download a PDF on my android phone and upload it to my paperless-ngx instance in a couple of clicks and easily edit the tags as necessary. It's great for travelling as you're not reliant on having a locally installed application on your chosen device with you, and of course it would still be available if you lost your main device and only had your phone on you.
Makes sense, but how about Dropbox / iCloud Drive as alternatives? PDFs / images are somewhat small (at least relative to videos). I just stuff all my PDFs in Dropbox. I'm almost completely paperless and I don't seem to accumulate that many scanned docs to fill up even the free tier storage space.
Yeah, it depends on what you want from a document management system. If you've got a bunch of searchable PDFs, then storing them in a cloud service might well be sufficient. Paperless-NGX adds OCR to the mix (probably more useful for scanned paper images) and also tags. When I add a document, it fills in the best guess for correspondent, document type and appropriate tags, which tends to be accurate for common documents (e.g. payslips, statements) and usually only needs me to change them if it's from someone new.
What I find most useful is grouping together holiday documents such as travel insurance, holiday booking details, passports etc. and assigning a suitable tag, so I can easily find the relevant info. You could easily replicate that by copying those documents into a separate folder for easy access, but with Paperless-NGX it does most of the organisation for you and the search is more flexible as you can specify what kind of document you're looking for and who it came from.