Alchemy is an image-to-PDF converter. Despite its name, it's hardly an arcane tool: it fits neatly into the menu bar of several other pieces of software, it provides drag-and-drop functionality, and it makes the conversion process simple and easy, with few steps. It can also convert image files into GIF format, or merge multiple images into one PDF.
Dozens Of Formats To Two
Little needs to be said about Alchemy's interface beyond how easy it is to access, being available from your menu bar. It is small and simple, and scales properly even to large 4K screens. It outputs GIFs and PDFs that look about what you'd expect from the base file, or from stringing multiple base files together in sequence. The only real snag is that it doesn't support every file format you might want: though all common, modern file formats such as PNG, JPG and TIFF are usable with Alchemy, some are not, including SAI files. Thus, to use Alchemy with Paint Tool SAI and other less common software, files need to be converted into PNGs first.
Simple Yet Strong
In essence, Alchemy is easy to use and will usually produce whatever you're imagining when you convert or merge files. That's really all it needs to do, so the lack of extra features is easy to overlook for anyone who just wants to turn their images into PDFs and nothing more.