You can see a surname on page three, yet the PDF search reports no matches. That result tells you what the search found, not necessarily what is printed on the page. A scan can look like ordinary text while containing only an image.
Start with the document open in the PDF viewer. Search within that file rather than the surrounding website or file library. In Acrobat, check options such as case sensitivity and whole-word matching before changing the document.
Try copying a short sentence
Select a visible sentence and paste it into a plain-text editor. Inspect the actual characters. Unexpected spaces, broken words or incorrect letters can explain why the exact phrase is not found. Try a distinctive short fragment and compare its result with the page.
If selection fails, the page may be an image, but tool choice or document permissions can also matter. Do not assume immediately that OCR is the answer. A mixed PDF may contain searchable pages and scanned pages, so one successful selection does not establish that the whole document is searchable.
Recognise text on a copy
For an image-only scan, use your application’s documented OCR feature if available. Adobe’s guidance provides page-range and language choices and recommends preserving the original. Availability depends on the application and edition.
After recognition, compare names and numbers with the visible scan. Search for several words you know are present, including on different pages, then save and reopen the copy. For an important short document, inspect the relevant pages visually too. Note the pages and methods checked; a single zero-results search is too weak a basis for saying that a term never appears.