Well, there's a few ways that you might be able to do this. The feature that I would try to capitalize on is that, in Adobe PDFs, the "sticky notes" are displayed as roll-over text (i.e., visible on mouse-hover). Creating these notes from Word, however (as you noted), is not easy. But here's some methods you might consider:
1. Easy (but not free):
You can do this automatically if you have access to Adobe Acrobat Professional version 10 or above. (The Acrobat software is not required to create a PDF, but it does provide more control over how your Word document is converted to PDF format.) Once AA Pro is installed on your machine, you will have an "Acrobat" tab on your ribbon. From there, choose "Preferences" and then the "Word" tab, and select the "Convert displayed comments to notes in Adobe PDF". Now, enter your text-to-hide as comments in the Word document, and convert to pdf. [Note: I have not personally tested this, as I don't have access to AA Pro on my machine. I got this information here and here.]
2. Free (but not easy):
If you don't mind a wee bit of manual effort you could do it this way:
- Type the text-to-hide in a white font. (Perhaps create a style to make this easier.) Use some symbol or other convention so you can easily find this text later.
- Convert to pdf
- Manually scroll through your pdf, find each hidden block of white text, select it,
Ctrl+C to copy it, right-click and select "Add Note to Text", then
Ctrl+V to paste. Click (with your mouse) outside the edit box to make it disappear. You will be left with a mouse-over note-icon (and a yellow highlight on your white text).
- Save the pdf.
3. Less easy (but equally free): There is probably a way to do a version of Option 2 above using VBA in a Word macro, using the hints posted in this thread, but I have not attempted to do so. I believe you can also do this using Open Office (from Libre Office), using the "Export Comments" feature. (I understand some people install Open Office just for its pdf conversion utilities.)