Preset quality, or an exact size?
Both tools compress a PDF for free, in a browser tab, with no software to install. The real difference shows up once you open each one: Smallpdf's compress-pdf page offers three fixed levels - Basic, Moderate, and Strong - and that's the whole interface. There's no way to tell it "get this under 2 MB" or "under 200 KB"; you pick a level, download, and check the resulting size yourself.
Compress PDF has that same level-based mode (Light, Medium, Heavy), but also a Target file size mode: type in 200 KB, 500 KB, 1 MB, 2 MB, 5 MB, or a custom number, and it tries progressively stronger settings until it finds one that actually fits - stopping at the first result under your target so it doesn't over-compress. That matters for the specific, common case of a hard limit - a job portal capping uploads at 500 KB, a government form at 2 MB - where "pick a level and hope" means downloading, checking the size, and starting over if it's still too big.
Where the file actually goes
Candid Tools runs the whole compression - re-rendering every page and rebuilding the PDF - in your browser using mupdf and pdf-lib. The file never touches a server. Smallpdf's compressor uploads the file to their servers for processing; by their own account, files are deleted around an hour after processing (longer for eSign flows), but the upload step itself happens either way. If a file never leaving your device is what you actually need - not just "eventually deleted" - that's the real dividing line, and it's covered in more depth, across more tools, on our private browser file tools comparison.
Where Smallpdf is genuinely ahead
This isn't a one-sided pitch. Smallpdf is a much bigger product: it covers dozens of PDF tasks (edit, eSign, convert to/from Word and Excel, organize, redact), has mobile apps, and integrates with Dropbox and Google Drive. Candid Tools doesn't do any of that - it's a set of single-purpose tools, each opened for one task and closed, with no account, no cloud sync, and no mobile app. If you need eSignatures or Office-format conversion, Smallpdf's paid plan (reported around $12/month billed annually - not independently confirmed on their site, pricing pages change) covers ground this site doesn't touch.
The actual tradeoff
Pick Smallpdf if you want one account covering the full range of PDF and eSign workflows. Pick Compress PDF if you specifically need to hit an exact file size without the file leaving your device, and you don't want an account for it. They're not really competing for the same job - Smallpdf is a PDF suite, this is one tool that does one thing precisely.