Why the size target matters
Application portals almost always cap the upload. NTA's JEE (Main) bulletin asks for category and PwD certificates as a PDF between 50 KB and 300 KB. USCIS online filing states that files must not be larger than 6 MB. Plenty of state portals stop at 100 KB. Go over and the form rejects the file, often without saying why.
Most converters have no idea what number you are aiming for. They produce a PDF, and you find out it is 4 MB when the upload fails. Here you set the target first and the converter searches for the largest image quality that still lands underneath it — and if quality alone is not enough, it steps the resolution down rather than crushing the picture into mush.
Why the file doesn't come back bigger than the photos
A common bug in image→PDF converters is that they decompress your JPEG and then re-compress the raw pixels with deflate, the algorithm inside ZIP. Deflate is very good at text and very bad at photographs, so an 8 KB photo can come back as a multi-megabyte PDF. This tool embeds the JPEG frame directly, exactly as JPEG, which is what the PDF format is designed for.
Photos from a phone come out the right way up
Phone cameras usually store the picture sideways and add an EXIF tag saying which way to turn it. Tools that ignore that tag produce sideways pages — and tools that half-handle it produce squashed ones, because four of the eight rotations swap width and height. This one reads the tag and bakes the rotation in before the page is laid out.
Nothing leaves your machine
These are ID scans, certificates and signatures. Every other converter asks you to upload them to a server and trust a retention promise. This page has no server to upload to: the PDF is assembled by about 12 KB of JavaScript running in your own browser. Open the network tab and watch — the page loads its own files and stops. Here is exactly how that is enforced.
All HermitFile tools
Every one of them runs in your browser. Nothing you open is uploaded.