Open and filter a CSV without uploading it

The CSVs people need to open in a hurry are exports: customers, orders, subscribers, attendees, payroll. Under most data protection regimes, sending a file like that to a third party is a processing decision that needs a lawful basis and usually a written arrangement with that party. A hosted viewer's terms of service are not that arrangement, and opening the file locally removes the question rather than answering it.

The practical ceiling is 100,000 rows, which covers most exports and not all of them. Above that, split the file or go back and narrow the export at source, which is generally the better move anyway: pulling every column of every record when you needed three columns of last month is the actual problem.

Hide columns before you start reading, not after. Most of what makes an export sensitive sits in a handful of columns you do not need for the question you are answering, and hiding them means a screen share, a screenshot or a colleague passing behind you sees far less. When you need to pass something on, export the filtered view rather than forwarding the original file.

Prove it to yourself once. Load the viewer, disconnect the network, then open a real export: sorting, filtering and exporting all continue to work, because the rows are being held and manipulated on your machine. A hosted viewer would have nothing to show you.