— From Mac OSX Hints
“If you have Stickies open, you can use it to quickly open and view PDF documents. Simply drop a PDF file into a sticky note and it appears inside the note with its own scroll bars constrained to the size of the note. If the sticky is translucent, then the PDF appears translucent too.
This hint works especially well with floating stickies (Command-Option-F).
Unfortunately, this trick only works with PDF files and not RTF or DOC files. If you drop any type of file on a sticky other than a PDF or textClipping, you only succeed in inserting a picture of the file’s icon.”