Scanning is not enough; pdfs must be OCR (Optical Character Recognition) which means that the pdf "image" is rendered into text and thus searchable.
Adobe Acrobat Pro has built in OCR.
OCR is also available more or less free elsewhere:
OCR is almost never 100% perfect.
For highest quality OCR and also foreign language OCR, try ABBYY.
Many scholars find scanner pens are excellent note taking devices.
Some also promise foreign language support, such as Edaco C Pens.