Pdf to DjVu Gui is a free portable tool - a front end for pdf2djvu - which converts any PDF document to the DjVu format.
DjVu files aren't great for wide public distribution as most people can't create, view or edit them, but they do have advantages, most notably that they can be much smaller (in our tests reductions of 25% to 50% were common, and we sometimes we achieved more). While this isn't just down to reduced resolution, the page quality is still reduced, but if you need to share very large PDFs with a small audience then it might be worth the tradeoff.
Conversion is simple but straightforward. Unzip the download, run "Pdf To Djvu GUI.exe", click "Add..." and select your PDFs. Choose your output options - most importantly, document resolution and page range, although you can also disable metadata, hyperlinking and other features - then click "Generate DJVU" and wait as your documents are created.
This can occasionally take longer than we expected, presumably depending on the source document (the converter hung on one page for several seconds). But we waited patiently, and conversion resumed every time, with the program creating its DjVu files in the same folders as our source documents (File.pdf > File.djvu).
DjVu documents aren't nearly as well supported as PDFs, but they're often considerably smaller, and if you can live with the reduced quality - and you're only sharing them with a few people - then they might be worth trying.