Converting a JPG to PDF is typically needed for submitting a photographed document (a signed contract, receipt, or ID photo) somewhere that only accepts PDF uploads. The photo becomes a single PDF page at its native resolution. For multi-page documents built from several photos, convert each one and then combine them with DuckConvert's Merge PDF tool.
A wrapper, not a compressor
Putting a photo into a PDF does not recompress it. The JPEG data is embedded as-is inside a page, along with the instructions describing where on that page it sits. The resulting document is therefore slightly larger than the image you started with, never smaller, and the picture quality is exactly what it was.
That is worth knowing because people sometimes reach for PDF hoping to shrink a file. It does the opposite. What it does buy you is a container: a fixed page with defined dimensions that prints predictably and that every document workflow in the world will accept.
Why so many forms insist on it
Passport applications, insurance claims, expense systems and university submission portals overwhelmingly want a PDF, even when what they are asking for is a photograph of a receipt or an identity document. The reason is that a PDF has a page size, so it prints and archives at a predictable physical scale, and it can hold several pages in one file where an image cannot.
That last part is usually the real requirement. If the form wants one document and you have photographed four pages, convert each and then combine them with the merge tool, rather than sending four separate attachments and hoping.