How to convert a JPG to PDF without uploading it anywhere
What actually happens when an image becomes a PDF page: the page size, the print DPI and the re-encode nobody mentions. With measured numbers from a real conversion.
Converting an image to a PDF sounds like packaging. Same picture, new wrapper. It isn't: the picture is decoded, re-encoded and given a physical page size, and each of those steps changes something the tools doing it rarely mention. Here is what actually happens, measured with our own converter, including the parts that make us look worse.
Does converting a JPG to PDF lose quality?
Yes, once. Your file is decoded to raw pixels and encoded again as a fresh JPEG at quality 92 before it goes onto the page, using the browser's own encoder through OffscreenCanvas.convertToBlob. The original bytes are never passed through untouched.
On a photograph you won't see it. Quality 92 is high, and one extra generation on a normal camera JPEG is invisible at any sane viewing size. Where it shows up is a screenshot full of small text, or a diagram of flat color with hard edges, because JPEG stores color at quarter resolution and puts ringing around sharp glyphs.
Why is my PDF bigger than the JPG?
Because a re-encode at 92 is usually a higher quality setting than your source was saved at, so the file grows even though nothing's been added. Three real runs through our converter:
A 32 KB image became 34.5 KB. A 449.9 KB image became 448.4 KB, roughly a wash. The 538.6 KB photo above became 674.2 KB. Nobody advertises image-to-PDF as a way to save space, and now you know why. If the file has to be small, run it through the image compressor first and convert the smaller version. Where that slider stops helping on a 3840x2160 photo is measured in its own piece.
Can I merge several JPGs into one PDF?
Not here, and this is the one place our tool does less than the alternatives. Drop in three images and you get three separate single-page PDFs, offered individually or as a ZIP.
Most of the big converters do merge, and if you need a single multi-page document, use one of them and accept the upload. We'd rather say that than imply a feature we haven't built.
Why does my converted PDF print blurry?
This is the question the popular pages answer with reassurance instead of arithmetic, so here's the arithmetic. The page is built in pixel units, and the PDF unit is the point, defined as 1/72 inch. The conversion factor in play works out at 4/3 of a point per pixel, so a 3840 pixel wide image produces a page 5120 points wide. That is 71.1 inches, about six feet.
Divide the pixels by that width and the nominal resolution is 54 DPI, and it is 54 for every image regardless of size, because the pixel count cancels out. That number is why "actual size" printing looks soft: you're asking for a six-foot poster.
The fix is the one your print dialog's already doing. Choose Fit to page, and that same 3840 pixel image lands on A4 width at roughly 464 DPI, which is sharper than any consumer printer resolves. Blurry output here is almost always a scaling setting rather than lost detail.
Is it safe to convert a passport scan or a bank statement?
Depends entirely on where the conversion happens, and it's the question the incumbents answer sideways. Every one of them uploads your file and then promises to delete it. Smallpdf says its converter "operates in the cloud" and deletes files one hour later. iLovePDF keeps them "for a maximum of 2 hours". Adobe says your file is handled by its servers and "deleted unless you sign in to save it".
Those are retention promises. They're answers about how long a copy of your passport sits on someone else's disk, not about whether it gets there. We built this one so the question doesn't arise: the decode, the re-encode and the PDF assembly all run in your browser tab, and there's no upload endpoint in the codebase to receive the file. Open your network tab during a conversion and watch it stay empty. Our privacy policy lists the only things we do collect, which is page views and anonymous tool-usage counts, and nothing else.
What happens to a transparent PNG?
Every transparent pixel is filled with white before the page is built. PDF as a format can carry image transparency; this converter doesn't use that path, and the page image is a JPEG, which has no alpha channel in any mode. A logo with a cut-out background comes out on a white rectangle. That's worth knowing before you convert a PNG to PDF expecting the transparency to survive, and which format keeps an alpha channel and which one fills it in is the longer version of the same rule.
The short version
Convert, then check the result at 100% if it contains text. Expect the file to grow rather than shrink. Print with Fit to page. And if the image is something you wouldn't email to a stranger, do the conversion somewhere that never receives it, which is the whole reason this converter works the way it does. If what you need is the words rather than the picture, reading text back out of a PDF is the job going the other way.