The documents most worth keeping local
PDFs are where sensitive material lives: contracts, invoices, medical records, identity documents. Every well-known PDF service uploads your file to process it. Here, merging, splitting, compressing, extracting text, adding passwords and removing them all happen in the page — the encryption tools in particular run qpdf compiled to WebAssembly, so neither the document nor its password is ever transmitted. Uploading an encrypted file and its password to a stranger's server in order to decrypt it is a strange thing to do, and yet it is the norm.
Photographs carry more than you think
Every photo your phone takes records the camera model, the exact moment of capture, and in most default configurations the GPS coordinates of where you were standing. Post it to a forum or attach it to a listing and you have published your location without knowing. The EXIF Viewer shows exactly what is hidden inside, then strips it losslessly — rewriting the file's container rather than re-encoding the image, so the picture data is byte-for-byte identical afterwards. Most removers re-save through a canvas, which strips metadata and degrades your photo at the same time.
Credentials and code should never be pasted into a form
A production password hash, a private key, a 2FA secret and an API token have something in common: pasting any of them into a website means a stranger's server has now seen it. The security tools here compute everything through the Web Crypto API in your browser. The same logic applies to code — SQL queries carry real table and column names, and JSON payloads carry real customer data, so the formatters and converters are local too.
Where local processing genuinely cannot help
Being honest about the limits matters more than the sales pitch. A browser cannot crawl your website to build a sitemap, or fetch another site's HTML to check its meta tags — cross-origin requests are blocked by design, so those tools take pasted input instead and say so. Some formats have no browser encoder at all: AVIF conversion was considered for this site and cut, because the browser silently returns a PNG when asked for AVIF, which would have meant handing people a mislabelled file. Local-first is a genuine advantage, not a claim that everything is possible.