PDFs can be sized to anything withing reason (not sure about the extremes of large or small, but I have done PDFs in just about everything from business card size to 4 foot by 8 foot posters. Depending on how they are created, since PDFs often start in other programs and are the results of a print driver that creates the PDF, the original program often has some limitations. For example, we have clients that want to make 4x8 foot posters, and do them in Powerpoint. But Powerpoint won't let you make a document with those dimensions. You can make the Powerpoint at 1/2 size (2x4 feet) but it will nor print out properly. You have to make it significantly smaller proportionally to get it to work. The printout issue can increasingly complicate things, as the Adobe reader allows printing output to scale in a number of ways, reducing or enlarging as needed to fill the printer paper with the image, printing to a fixed percentage of the size or printing to the original size and "tiling" the printout if the document page size is larger than the printer paper (so each page would be partially printed on multiple pages that can be taped together to make a larger piece.
I'm not entirely clear on what you're trying to do, but if, for example, I wanted to make a "business card" size document for others too print out, I would make a standard page size (i.e. letter paper, 8.5x11) with the document repeated on the page as often as possible to use the paper, so a single page might have 4, 8, 10, whatever number would fit and blank space to make up the difference. That way the user could print it out on standard Letter paper (probably 98% of printers can do this size) and get the closest approximation of the original size (every printer has slightly different margins, so if scaling is selected it may be printed within 95-105%).
As for editing PDF files, it can be done. You typically would need the full version of Adobe Acrobat or another program that can edit PDFs (Adobe Illustrator, InDesign, Photoshop, probably some others), but there are issues with it, mostly around the conversion process between the original application (Word, Powerpoint, whatever) and the PDF File. It often will create random breaks in the text of the document, so if for example the document had the following line:
"The quick brown fox did something I can't remember."
the PDF might have the text broken into the following pieces:
"The qu"
"ick brow"
"n fox did something I c"
"an't rem"
"em"
"ber."
and then those text boxes are laid out on the page so it looks just like the original. However, if you try to change "quick" to "fast" you run into problems because the text is in 2 different fields, and you may or may not be able to move them, so removing or adding text causes REALLY strange headaches.
Now if you use the Adobe apps that use PDF as their format, Adobe Illustrator for example, you don't get this problem, because the print driver conversion process is not used. But that's not necessarily a good solution, because PDFs also use a funky system to replicate any unusual fonts you may or may not be using, so you still may get font errors when trying to edit them.
There is a method for making "forms" in PDF format, that are intended for users to fill out, and have some areas where text can be added, but I have not dealt with these too much to know how flexible they can be with design and such. All the ones I have worked with were just fields to fill in your name/address/etc. Not sure if something like that would work for your ideas.
I have seen some examples of Adobe Flash apps that can do these sorts of things on a web page, with formatted text that can be edited and moved around within a fixed design layout and then printed, but I'm not sure of any easy way to make it something that could be saved by the end user (unless of course they had the PDF print driver) for future use, and saving them to your web site would require a whole bunch of Database know-how, which is beyond my understanding...
I've been looking at doing something similar to that in Adobe Flash, so if I get some time and feel like playing I may be looking into setting something like that up, if I find out anything I'll post it...
(hope I didn't go to geeky for you...)
While I was typing all this giberish, the Word option came out. Another option in that end is to save it as HTML from Word. Most Word versions back to 2000 or so will edit in the HTML format without having the version compatability issues, and allowing editing in other applications like Dreamweaver or even Text...
