Will the photos be compressed again?

No. JPGs are embedded unchanged, so there is no second round of quality loss.

Use Images to PDF, which takes any mixture.

Why is my PDF so large?

Because the photos are. Compress the result if it needs to be smaller.

convert

Convert JPG to PDF

Combine JPG photos or scans into a single PDF.

Runs in your browser

How it works

JPGs are placed into the PDF exactly as they are, with no re-encoding, so nothing is lost and the file is no larger than the images you started with. This is the right tool for phone photographs of paperwork and for scanners that produce a folder of images instead of a document.

Related tools

PDF to Images

/pdf-to-images

Merge PDF

/merge-pdf

Compress PDF

/compress-pdf

Questions

Will the photos be compressed again?: No. JPGs are embedded unchanged, so there is no second round of quality loss.
Can I mix in a PNG?: Use Images to PDF, which takes any mixture.
Why is my PDF so large?: Because the photos are. Compress the result if it needs to be smaller.