Question: How can I merge several `.png` files into one PDF file in Unix?

Answer: 
From looking through the documentation on ImageMagick, it might be as easy as:

```
convert 1.png 2.png myfile.pdf

```

See comments about possible risks. If that doesn't work, [PDFjam](http://www2.warwick.ac.uk/fac/sci/statistics/staff/academic-research/firth/software/pdfjam) claims to be able to solve your problem.
If I want to merge some scans to one PDF file, I do this:

```
pdfjoin --a4paper --fitpaper false --rotateoversize false scan01.png scan02.png

```

This gives you a PDF document with DIN-A4 page size, where every `png` file is centered on it's own page. Images that are too big for one DIN-A4 page are resized proportionally to fit on one page. Smaller images are not resized (not made bigger).

You have to name all `png` files on the command line, but you can also use wildcards to eg merge all png files in the current directory:

```
pdfjoin --a4paper --fitpaper false --rotateoversize false *.png

```

The `pdfjoin` command is part of PDFjam as mentioned in the answer by Jeremiah Willcock. So you will most likely have to install a package named `pdfjam` or `texlive-extra-utils` with your distros package manager. PDFjam is able to use `png` files as input since Version 2.07, released in 2010-11-13.