There is no easy way, however, here are a few suggestions that you might employ.
[ol]
[li]If you scan them as an image and save them as PDF's they are not searchable.[/li]
[li]Though very time consuming, one could scan the old documents, then by utilizing OCR software to convert to a text file, edit with word processor, insert the scanned photographs, charts, et cetera then print as a PDF.[/li]
[li]Or a hybrid of the aforementioned methods. Scan as images; insert the images into a word-processing document. The word processing documents could contain key words, your notes, et cetera, that would be searchable.[/li]
[/ol]