From the document that Matt points to above:
Summary:
Users get lost inside PDF files, which are typically big, linear text blobs that are optimized for print and unpleasant to read and navigate online. PDF is good for printing, but that's it. Don't use it for online presentation.
That is both right and wrong. Where a PDF is large, I will not argue, but where it is just one or two pages maximum, PDFs are a valid form of presenting information. Don't dismiss something because it is being abused by a lot of authors. I suggest we must also acknowledge that PDFs were originally developed to provide security that most word processing documents of that time just could not offer. They were also used as a method of transporting documents that a lot of printers could take and use as printable documents - or so I am given to understand.
In the event of a PDF of several mb and larger, I would suggest that, in Moodle at least, a Book would be a far better option. If for anything other than Moodle, there is the possibility of a *.chm file for Windows, which can be viewed in Linux with KchmViewer on KDE, ChmSee for Gnome or the CHMViewer plugin for all flavours of Firefox. How this works with the newer *.hxs help file types, I just do not know. This, of course, assumes you are interested in using a product from the Dark Side...
Yes, I do accept that using Word, or Indesign, or Excel, or Acrobat Pro to create a PDF is probably a lot easier than using most other tools to protect the inegrity of your work. Just be aware, bigger does not always mean better. Breaking larger files down into smaller, more specific topics and then turning each one into a smaller PDF presents a better outcome that having just one large PDF.