Unable to find a word in a pdf document which is searchable in Pc or laptop

If it is happenig with some PDFs maybe you'll need to verify if these PDFs were produced from a scanning device.

If that was the case, I would run the Accessibility Checker tool with Acrobat Pro DC(in your dektop verion) and the report will indicate what exactly are the problem with that file.

I am emphasizing my inquiry about the possibility of a scanned document that was converted to a PDF because the text is rendered as an image layer on top of another image layer. Visibly on screen we are able to read what appears to be real text, but the produced PDF has no searchable text and images. Or even worse, no appropriate tagging structure (this is just a guess, I may be wrong). 

You may need to run the "Scan & OCR" tool on those files to manually and attempt  to have Acrobat perform a an OCR text recognition (make text and images searcheable). Additionally also try the "Print Production" tool --->>> "Preflight"--->>>"PDF Fixups" --->>> "Analyze & Fix".