Great question! I found this guide very helpful: https://forums.adobe.com/thread/1008278
Key points from this guide:
PDF Optimizer (Learn how to reduce size, remove fonts embedding, remove unwanted data etc)
Acrobat 9
http://help.adobe.com/en_US/Acrobat/9.0/Professional/WS58a04a822e3e50102bd615109794195ff-7 c84.w.html
Acrobat X
http://help.adobe.com/en_US/acrobat/pro/using/WS58a04a822e3e50102bd615109794195ff-7c88.w.h tml
Conversion Process:
To create smaller size PDF, I would suggest to plan it from the beginning. As Dave mentioned- smallest size PDF, you can select this joboption from the conversion dialog box (If you have the origional document word etc.which you are creating a PDF from). This will automatically do maximum part of the job for you. So, try to keep the size lower from the beginning of PDF creation won't give you much trouble later.
Images:
You can also create small thumnails of the images and insert them in the origional document before conversion. do not resize them in the document as it still contains the origional size. Try to compress them or resize them outside the document and then finally insert them.
For example: if the image is 640x480 and you want only 50x50 in the PDF document. Try to resize it using some applications like Adobe Photoshop or MS Office picture manager (comes in MS Office suite) or using many apps freely available online. then finally insert them in the document. If you resize them in the document itself, they will still be containing the origional size.
Along with this, try downsampling the images to 72ppi (using Acrobat PDF optimizer) and use poster images instead of the videos. These images can have links pointed to the videos over website.
PDF Size:
Try to keep the PDF size between 1-2 MBs.
I hope that this helps! Good luck to you