How to make Spotlight index PDF documents to search faster or on my command?
Spotlight indexes are updated when a file is written, so results should be available within milliseconds of the write operation being finished.
Delays in finding a search result are due to a slow or problematic index or a failure of the scanner to complete that file.
You'll need to diagnose spotlight or perhaps just rebuild your index around the folder if you have problems with the subsystem. It's hard to know what of the three causes is happening without someone who knows how things work poking at the system.