What's in the file? Mostly text? A lot of images?
For images:
• Consider lower resolution or smaller placed size (instead of 6" wide, put them 4" wide on the page)
- images for use on screen need to be no larger than 150 ppi; typical screen resolution is 72 ppi and if they zoom in at all (small documents on large monitors) you can go higher. I find 100 ppi to be a nice compromise but your usage may need something else
- images for print will experience dramatically diminishing returns above starting around 200 ppi and as a rule of thumb nothing needs to be higher than 300 ppi.
• Look at compression settings in your PDF creation settings. Higher compression means smaller file size, but can degrade image quality. Higher resolution images are more resilient to the effects of heavy jpg compression, but are not immune. Low resolution, high compression creates the worst image degradation.
• Depending on the source application, using transparency over images may create unnecessarily large files. This includes placing layered images or using many effects (drop shadows, transparent type or other elements) in the host application.
On type:
• If many typefaces are used, this can very quickly inflate file size. In PDF options, you can modify font embedding as appropriate. Some fonts are as large as 2 MB (or larger?), though most are much smaller.
- Fonts installed on machines where the document will be viewed does not have to be embedded. Be warned, however, any font NOT embedded will NOT display properly if the host machine
- Font subsetting should be always used, as a rule of thumb. This will only embed the glyphs that are used in the document. (If only a,b and C are used, only those letters are embedded)
Wall of text.
I work in print advertising and deal with PDFs every day. If you have any questions, feel free to PM me. If you have a non-sensitive document you'd like me to examine, I can tell you where there might be other opportunities to save on file size (some applications include a shitton of metadata that isn't used, for example).