PDFs Not Being Completely Indexed

Forum member "ttw" noticed the following happening in Google:

"I was checking to make sure that all my client's PDFs were appearing in Google's index and noticed something odd. Luckily all the PDFs appear when I do a site:yoursite.com filetype:pdf.

"What's odd is that sometimes -- depending on the search phrase that I use to bring up the PDF -- the PDF may or may not appear in the SERPs."

If anyone can explain what might be causing this, please add your comments to the thread here.