I spoke to Apple Tech Support and they fixed the issue for me on my 2014 MacBook Air. This is the conversation I had with Tech Support, but the solution for mine didn't work on my sister-in-law's 2008 MacBook Pro. Please take a look and suggest, if you have time, what the problem might be with hers considering that it worked on mine:
06:36:25 AMJohnThanks for contacting AppleCare chat support. My name is John. How can I help you?
06:36:45 AMJohnHi James. I see that you are having problems finding files in Finder, is that correct?
06:38:19 AMJamesMy problem is that I remember, under Mavericks and mountain lion, being able to search for both files and for key words within word files without them being opened. I have lots of business invoices, but the invioice numbers were part of the word file text. They were not in the file name. I was able to find them by typing the invoice number into Finder. Now I cant do that under Yosemite
06:39:35 AMJohnOkay James. I can definitely understand this being an issue. If you wouldn’t mind I would like to ask you a few questions about this.
06:39:46 AMJohnMay I ask how long ago you installed Yosemite?
06:40:41 AMJamesAbout three weeks ago. I have a friend who installed it yesterday and she's having the same problem.
06:41:12 AMJohnI just tested by typing in the name of an Advisor on a spreadsheet and I have actually had the correct results in doing so.
06:41:20 AMJohnSo I think that we may have to do a few things to resolve this.
06:41:27 AMJohnMay I ask if you know how to get to Disk Utility?
06:41:51 AMJamesIs it startup disk?
06:42:12 AMJohnI do not believe so. May I ask if you can click on Finder > Applications > Utilities ?
06:42:22 AMJamesok, one sec
06:42:37 AMJohnOnce there you should see Disk Utility. I need you to open that and then let me know once you are there please.
06:43:28 AMJamesgot it'
06:44:02 AMJohnThank you. Now what I would like you to do is click on the indented Mac HD on the left please.
06:44:10 AMJohnThen on the right select “Repair Disk Permissions” please.
06:44:37 AMJamesdoing it now
06:44:45 AMJameswaiting for it to complete
06:45:14 AMJohnAlright thank you.
06:45:22 AMJohnHow much time does it show that it needs, if I may ask?
06:45:29 AMJamesA minute or less
06:45:52 AMJohnAlright awesome. A minute left generally means that there is a few minutes left.
06:46:03 AMJohnBut you have a flash drive so you should be good to go.
06:46:25 AMJamesok its done
06:46:29 AMJohnAlright perfect.
06:46:35 AMJohnNow if you do not mind click on Verify Disk please.
06:47:58 AMJohnThank you. Now if you can click on the Mac HD above that (or the 128gb ___ drive) and click on Repair Disk please.
06:48:39 AMJohnAlright perfect.
06:48:47 AMJohnSo go ahead and close out Disk Utility for me please.
06:49:06 AMJohnThen click on the Apple and Select System Preferences please.
06:49:28 AMJamesok done
06:49:43 AMJohnOnce we are in System Preferences I would like you to please click on the Spotlight option and then choose Privacy.
06:50:15 AMJohnNow before we go any further there (leave it up as we will need it in a moment please.) open Finder and then choose Finder by the Apple and select Preferences please.
06:51:03 AMJohnOnce we are in preferences, please select Sidebar and at the bottom, ensure that Hard Disks has a Check mark in it please, instead of a dash.
06:52:37 AMJohnOkay then please close out the finder preferences and Finder
06:52:38 AMJohnThank you
06:52:52 AMJohnI hand type out these directions, sorry.
06:53:08 AMJamesFine, I've got plenty of time
06:53:23 AMJohnOnce that is closed, please go back to the Spotlight window and click on the + sign in the bottom left of the Privacy Tab.
06:53:56 AMJohnOnce you do that, on the left hand side of the Finder window that appears Select Mac HD and then click on (Choose) in the bottom right please.
06:54:25 AMJohnThis should select the entire Mac HD and then place it in the white area above the +/- signs
06:54:58 AMJohnAlright, now please highlight the Mac HD in the space above and then click on the - sign.
06:55:30 AMJamesSo remove what I have just added?
06:55:35 AMJohnThis will force the Disk to become searchable again in Finder. What is going to happen is that finder now has to Index the Drive and ensure that all files/folders etc are searchable.
06:55:38 AMJohnYes, please.
06:56:10 AMJohnBy forcing it to disable / enable searching it should allow you to be able to Search the way you used to.
06:56:33 AMJamesYou want me to close everything now?
06:56:38 AMJohnJust to ensure that we have taken every step possible, I know need you to close that window.
06:57:05 AMJohnThen open Finder > Click on the Gear in the Center of the Border and choose Show View Options.
06:58:23 AMJohnSorry James, I meant to say choose Show Search Criteria, not View options
06:58:53 AMJohnThis way when you search you can look specifically for the Invoice Numbers.
06:59:01 AMJohnYou may also be able to type it in and just find it now.
06:59:50 AMJamesok, seems to be okay now, was it a change in Yosemite?
07:00:05 AMJohnNo, it was the way your drive was able to search.