I don't know why after 52 answers some one still finds it valuable to let me shine a light on this. There are already plenty of good suggestions among them.

But I do would like to add that the word "convert" is not exactly right. In fact, the JPEG is being packaged into a PDF format, and while doing so, some extra settings can often be chosen to re-compress the JPEG even further, resulting in an even smaller filesize. Of course, the 8 x 8 clusters of pixels will be rendered more coarse.

Another aspect is that the PDF is absolute about the physical size of each page. So the resolution (number of pixels) of the JPEG will finally get a width and height as a definite size of the page it fully sits on. Some tools offer an option to set this size beforehand, some don't and just assume a size, based on a 72 dpi factor or whatever the JPEG is stating (if it does at all).

But in now way the JPEG in the PDF will start to look better, yield sharper details, or be able to scale to any size you need (at least not without looking ugly).

Oorspronkelijk geplaatst op Quora: How do I convert JPG to PDF?"