This is the second question I've had in my last two shifts about Powder Diffraction Files, so I would bet that there's an assignment out there. If someone comes up to the desk asking about how to find PDFs, they might not be talking about online articles. :) They're looking for a set of Powder Diffraction Files that are located at QC482 .D5 P63. They might already have the PDF number (e.g. 37-453), but if they don't, there are indexes, such as the Hanawalt Search Manual (QC482.D5 P633).
If the patron is looking for set 37, and it looks like the series ends at 36, look around. Sets 1-36 are split into two series, one for inorganic and one for organic. After 37, they're combined in a set of red volumes. All three series have the same call number: QC482 .D5 P63
I hope this saves you a little time searching!