How to create single PDF from Preview (avoiding drag and drop)

Hello,

I am doing an online course and I have slides that I would like to merge into a single PDF.

I have taken snapshots of all the slides and these are ordered correctly by a timestamp so they are in the correct order. Question - how to get these 250 picture files into a single PDF?

I have opened them in Preview and selected all, then exported as a PDF but my Mac thinks I want to export 250 separate PDFs and littered my desktop with individual files.

The only workaround I can find is to import them into Keynote and export them as a PDF that way, but I end up with a background since the picture files do not cover the entire page, so unless I go into each one individually, its going to take too much time. Alternatively, I could do this in Preview, but I would need to drag and drop each one individually, from what I understand, and I want the files to stay in order and not duplicate, which is really to mess up with that particular method.

What I really need is a way to import all these PNGs without the page break into preview; or a way to import the files and remove the page breaks, then export as a single file...

Does anyone please have any idea? Seems a simple request but difficult to practise it.

Many thanks,

M

PS - then trying to get the Airdrop to work and recognise my iPad is another question ...