Does converting JPG photos to PDF reduce their sharpness?

What byte for byte means

PDF can carry a JPEG in its original encoding. Images to PDF takes advantage of that: it takes the file from your device and places it on the page without decoding it or compressing it again. If you extracted the image from the PDF later, you would get back the same data you put in.

Many tools work differently: they decode the photo and recompress it at a quality they choose to make the file smaller. Each such conversion eats some detail, and with documents the small print can blur. That does not happen here; the price is that the PDF never gets smaller than the images added together.

The exceptions, stated plainly

• A JPEG with an EXIF orientation other than 1: PDF ignores that tag, so the photo is really rotated and written again as JPEG at quality 92. A small loss, and a note next to the file name says so.
• A WebP image: it cannot go into a PDF, so it becomes a lossless PNG if it has transparency or a JPEG at quality 92 if not, and that is noted too.
• A PNG image: embedded losslessly, so the pixels stay exactly as they were, transparency included.

If no note appears next to an image in the list, it went in unchanged.

So why does a photo sometimes look softer?

When someone complains of blur after converting, the cause is nearly always one of three things, none of them related to encoding:

1. The viewer is showing a quick preview: phone PDF readers draw the page fast at low resolution, then refine it. Zoom in, wait a second, and the sharpness returns.
2. A small image stretched over a big page: an 800 px wide photo across a whole A4 page works out to about 73 pixels per inch and looks soft in print. The limit is the original photo, not the conversion.
3. The photo was already compressed: a picture saved from a chat app or a website may have lost detail before it reached you. The conversion carries it over as it is, flaws included.

How to check it yourself

Compare the size shown next to the image in the list with its size in your file manager: with no note, they match. Then open the finished PDF and zoom into fine detail, such as small print on product packaging, and compare it with the original at the same zoom. If you need a smaller file, the right move is to compress the photos yourself first with the Image compressor at a quality you have seen and accepted, rather than letting a converter decide for you.