PDFs are great, but there are a couple of issues. The biggest problem is that PDFs have this incremental update feature, which means, basically, that every change you make is stored in the PDF. The second problem is that everybody takes their pictures at 300+ dpi when, in fact, the printer, monitor, and you eye can’t tell 300dpi from 72dpi. Instead of using whatever tools you use to save the PDF, see if there is a Save optimized option (there is in Acrobat Pro). Often, just choosing Print to PDF is sufficient. If you can, downsample the images to 72dpi. Go through and discard all the useless stuff that PDFs save: alternate images, page thumbnails, bookmarks, search indices, comments, metadata, file attachments.
The current limit was not just a random value. The web team looked at EVERY large PDF that has been submitted over the past three years (some over 8MB) and the largest file after running an optimization was about 1.1mb, verifying that there was no visually obvious degradation. We then added 50%.
If your publishing tools does not have an optimizing tool, try searching the web for "optimize pdf." There are a number of free or online products that will do this. If you're sending your newsletter out via newsletter, you should be doing this already.