What changes when you convert JPG to PDF
This is the cleanest of the image-to-PDF conversions, because a PDF can hold JPEG data natively. The bytes of your photograph are copied into the document without being decoded and re-encoded, which means the image in the PDF is bit-for-bit the image you started with. No quality is lost in the transfer, and the PDF is roughly the sum of the JPGs plus a small amount of structure.
Each photo gets its own page at its own aspect ratio, scaled to fit rather than cropped. Where several images are converted together, the staging list sets the page order. That ordering is usually the entire point: an expense claim lists receipts by date, and a document photographed over three sittings has to come back together in the right sequence.
Photographs shot in portrait and landscape produce a PDF with mixed page dimensions. On screen this looks correct; on a printer it is awkward, and a print shop may charge for the inconsistency. If the output will be printed or filed against a form that expects a standard size, cropping the sources to a consistent ratio first is worth the extra minute.
iPhone photos usually arrive as HEIC rather than JPG, and a PDF cannot hold HEIC image data at all. Those have to be converted to JPG first — it is one extra step and it is a property of the PDF specification, not a choice made here.
There is no OCR. The result is a document containing pictures of whatever you photographed, which is fine for receipts and identity documents and wrong if you need the text extractable. A phone scanner app that adds an OCR layer at capture time is the right tool when searchability matters, because that layer cannot be added convincingly afterwards.
File size follows the sum of the inputs, because nothing is compressed on the way in. That is deliberate: a receipt photographed at full phone resolution stays legible, and a JPEG re-encoded twice starts losing the small print that was the reason to keep it. If the finished PDF is too large for an upload form, compress it afterwards as a visible, separate decision rather than degrading every page silently before it goes in.
This page is the JPG-to-PDF case specifically. The general tool, with its full set of limitations and questions, is Images to PDF.