Where the file goes
Upload-based: the complete file is copied to a server. It exists there in readable form for at least as long as processing takes, and usually longer — temporary storage, a queue, a cache, a backup, a log. Reputable operators delete it on a timer and say so. That statement is a promise about behaviour, and it is only as good as the operator, their security, and whoever can compel them.
Browser-based: the file is read into the tab's memory by the File API and never transmitted. There is no server-side copy because there is no server-side. The claim is not a promise about behaviour; it is a property of where the code runs, and you can confirm it in your browser's Network tab in about fifteen seconds.
That difference is worth little for a holiday photograph and a great deal for the documents these tools are actually used on: payslips, tenancy agreements, passport scans, medical letters, bank statements, board minutes.
Speed, and who is doing the work
Upload-based work is paced by your connection twice — once up, once down — and then by a queue you are sharing. On a fast connection with a small file, that is a couple of seconds. On hotel Wi-Fi with a 40 MB scan, the upload alone is the whole experience, and it is the part that fails.
Browser-based work has no transfer at all, so a small job is effectively instant. What it does have is your device's processor and memory, which is where the honest limitation lives: a very large document processes faster on a laptop than on a five-year-old phone, and a rented server with a lot of RAM will beat both on genuinely heavy work.
The crossover is roughly: for the everyday case, local is faster because there is no transfer. For the extreme case — a 500-page scan, a batch of a thousand images — a server wins on raw capacity.
What each one can do at all
Some operations need resources a browser tab does not have. Optical character recognition on a long scanned document is the clearest example: the recognition engine is large and the work is heavy, and doing it locally means downloading several megabytes of WebAssembly and then waiting. It is possible, and it is a real trade rather than a free win.
Equally, some things only the local model can do. It works with no connection at all, which no upload-based tool can. It has no page cap, no file-size tier and no queue, because there is no shared resource to ration. And it cannot leak your file in a breach of a service you signed up to, because you did not.
Where upload-based is genuinely the right choice
Collaboration and audit. If several people need to work on the same document, or if the process needs a legally meaningful record of who did what and when, that requires a server holding shared state — a local tool cannot provide it, and pretending otherwise would be silly.
Cryptographic signing with identity verification. Binding a real, verified identity to a document requires a certificate authority and an infrastructure that is inherently remote.
Very heavy batch work. Thousands of files, or documents in the hundreds of megabytes, are a fit for hardware you rent rather than hardware you are holding.
Integration. If the file needs to land in a document management system, a CRM or a workflow, the server-side model is the one that connects to those.
How to tell which one you are using
Open developer tools before you start, switch to the Network tab, and run the operation. If a request appears carrying megabytes of data outbound, your file was uploaded. If the traffic is only the page's own scripts and then nothing, it was not.
A second, cruder test: turn off your connection after the page has loaded and try to use the tool. A local tool keeps working. An upload-based one cannot.
The short version
Use a local tool for anything private, anything routine, and anything you would rather not queue for — which is most day-to-day PDF and image work. Use a server-side service when you need collaboration, verified signing, integration, or capacity beyond what the device in your hand has. The mistake is not choosing one; it is not knowing which one you chose.
