An app where I can select some pictures and turn them into PDF.

Add the images to a document/presentation in LibreOffice Writer or Impress / Microsoft Word or PowerPoint / Apple Pages or Keynote.

These all have an Export (or print) to PDF function.

1. Install ImageMagick
2. cd your-directory (replace your-directory with the directory where the images are)
3. convert image001.png image002.png <...> combined.pdf (replace image001.png etc with the list of your images)

This is very straightforward on macOS, if you have a Mac. Select the files you want in Finder, then right-click on them or click on them while holding the Control key, and in the menu that pops up look for Quick Actions > Create PDF. You can then rearrange things or otherwise edit the file in Preview (add text, etc).

https://support.apple.com/en-euro/guide/mac-help/mchl21ac2368/mac

Sounds like a slightly cursed situation to need to convert a folder of images stored in an open format into a singular PDF (a famously proprietary format that Adobe makes it intentionally difficult to work with outside of their software)

Easiest is probably to toss the images into Word or Libre Office Writer, arrange them as needed and export as PDF (in word select "XPS" as the export option which bypasses Acrobat license requirements)

If you instead want each page to have different dimensions appropriate to the image for that page I'm not aware of any software that does that other than a paid version of Acrobat (although I wouldn't be surprised at all if any do exist)