For Adobe Reader, go into Preferences:
Edit > Preferences > Search
The Fast Find option is enabled by default (with a 100 MB Cache).
This makes future searches faster by storing information about PDFs used often.
"Find" checks each word against the "Find" query. The first go, it takes a while for large PDFs.
Until "bumped" out of the Cache, subsequent "Find" of a document is faster.
With Acrobat Pro one or more PDFs can be processed into a Catalog Index which supports advanced Search (and faster response to a query).
Alternatively, an individual PDF can be provided with an embedded index.
Either index type can be utilized by Adobe Reader.
thanks for your answer, i knew it couldn't be that big a mystery!
i did the experiment yesterday: i did several search in a big document, and everytime it started to look at every page slowly. Before going to bed i did a last search, it was still looking at every page, so i stopped the process immediately. I left the computer switched on with reader running and the document open, (but no operation was processing), i just left everything like it was and went to bed. Today when i woke up i went to my computer, typed a new search in reader, and it found the word immediately.
my big wonder, because i'm a curious kind of person, is what happened during the night? reader said to itself "ok now we're alone, nobody's asking me this or that, i have plenty of time to analyse this whole document and fill up my cache memory before the guy wakes up"?
it looks like i just have to let reader run without asking anything, and after a while it knows every word in the document, how can it possibly do that?
i'm just wondering, i like to understand how things work... if you have a minute to explain to me!