Try choosing a Profile BEFORE scanning. I too at first found that the option to save as TXT was gone once I had scanned and was about to save. I also found that I have to scan in colour, not black and white or greyscale.
Open HP Scan. Click on HP Scan Profiles (top left). Then, with the HP Scan Profiles selected, select the Documents profile in the window right next to it.
Duplicate this Profile by clicking on the arrow next to Profile settings option at the bottom of the window. A version of that profile will now be selected and Custom Profiles will be selected instead of HP Scan Profiles.
So you should now be working on a copy of a Documents Profile under the Custom Profiles Tab.
Set this before the scan button is hit.
Rename it OCR Documents, then amend the Profile settings at the bottom left of the window as displayed in the window itself - that is choose "Resolution - Optimise for text recognition (OCR)". It's under the "Capture" tab by the way.
After Scanning, choose SAVE. You should save as TXT or RTF.
My version of HP Scan is 2.1.3 (7), Copyright © 1999 - 2009, Hewlett-Packard. My printer is C7200 series so the software may be different for your 7520. On the other hand it may not.
If you still cannot find the option, its possible that you may still have the installer .pkg for HP Scan v4.0.50 (or know where to re-download it). If so, you could try moving it somewhere safe then downloading my version of HP Scan and try running that.
Message was edited by: El Deanio
I realise that I haven't tried searchable PDFs with HP scan yet. I'll have to try that out tomorrow night.