Use ImageMagick to combine multiple PNGs into single PDF

At its most basic, without any Powershell or BATCH commands, you can stack 3 images one above the other, without spaces and resize to an A4 page like this:

convert image1.jpg image2.jpg image3.jpg -append -resize 2480x3508 page-01.png
convert image4.jpg image5.jpg image6.jpg -append -resize 2480x3508 page-02.png
...
...

Then combine all the pages into a PDF like this:

convert page-* result.pdf

If you have more time and patience, you can space the images out a little, or automate the process, but it may not be worth the effort.

If you want to space the images by, say 10, pixels, you can create transparent spacers in between like this:

convert -background none -size 10x10 image1.png xc:none image2.png xc:none image3.png -append -resize 2480x3508 page-01.png

In case any more Windows-y (TM) folk look at this, and feel like converting, you could do it like this on Linux/Unix:

Generate a file with all the filenames you want in the PDF, 3 to a line:

ls image*.jpg | xargs -n3 > files.txt

which gives:

i-1.png i-10.png i-11.png
i-12.png i-13.png i-14.png
i-15.png i-16.png i-17.png
i-18.png i-19.png i-2.png
i-20.png i-21.png i-22.png
i-23.png i-24.png i-25.png
i-26.png i-27.png i-28.png
i-29.png i-3.png i-30.png
i-4.png i-5.png i-6.png
i-7.png i-8.png i-9.png

I presume you would use DIR /B | something on Windows for that.

Then read that file, line by line and make a page of A4 sending it to stdout for a final convert to assemble into a PDF:

while read names ; do
   convert $names -append -resize 2470x3500 +repage miff:-
done < files.txt | convert miff:- result.pdf

I presume you would use FOR /F ... to do that on Windows, something like this mess:

FOR /F %%G IN (files.txt) DO (
   convert %%G -append -resize 2470x3500 +repage miff:-
) | convert miff:- result.pdf