Two very different architectures share one name
Server-side converters take your file over HTTPS, run a headless copy of LibreOffice or a commercial SDK on their infrastructure, and hand back a PDF. Your data sits on a third-party disk, however briefly, and passes through their logs, their queue and possibly their CDN.
Client-side converters ship the conversion code to your browser instead. The file is read from disk with the File API, parsed in memory, and written to a PDF locally. No request carrying your data is ever made.
Both are described as 'online converters'. The difference matters enormously and is almost never stated on the landing page.
What 'files deleted after 1 hour' actually means
It means the primary object is scheduled for deletion. It usually says nothing about backups, about the reverse proxy's access logs, about the error-tracking service that captured a failed conversion, or about which jurisdiction the disk is in.
None of that is necessarily sinister — it's ordinary infrastructure. But it does mean 'deleted after an hour' is a weaker guarantee than most people read it as, and it's not a guarantee you can independently verify.
How to check whether a tool uploads your file
You don't have to take anyone's word for it. Open your browser's developer tools, go to the Network tab, and convert a file. Watch for a POST or PUT request with a payload roughly the size of your spreadsheet. If you see one, the file was uploaded.
The blunter test: load the page, disconnect from the network, then try to convert. A client-side tool keeps working. A server-side one fails immediately.
- Network tab: look for a large POST/PUT when you hit convert.
- Offline test: disconnect, then convert. Client-side tools still work.
- Check whether a 'processing…' spinner is doing real work or waiting on a server.
When server-side is genuinely fine
Public data, marketing material, anything you'd publish anyway — the risk is negligible and server-side tools often support more formats and features, including charts and images that client-side tools can't yet draw.
The line is worth drawing at data you'd have to disclose if it leaked: personal data, financial records, health information, anything under NDA. For those, a converter that never transmits the file removes the question entirely, because there's no transfer to assess.
What about your organisation's policy?
Many DLP policies prohibit uploading company data to unapproved third-party services, and an online converter is exactly that. A client-side tool sidesteps this because no data leaves the managed device — but confirm with your own security team rather than assuming, since some policies restrict the sites themselves regardless of architecture.