How shall I grep a multi-line pattern in a pdf file and in a text file?

In the output of less my.pdf, a string `image not available` appears multiple times, for example: ... Lastly, what remains to ^L image not available ^L Implementations and Systems I would like to grep the string in the pdf file, for example, by pdfgrep. But pdfgrep -ni "image not available" my.pdf doesn't find anything. What shall I do then? Related question: Since pdfgrep has similar user interface to grep, how does grep handle a pattern which spans more than one lines? Thanks.