I had something like that happen to some PDF files. A new girl in my office scanned some manuals and did a good job with the OCR but forgot to edit and add the diagrams back so I had many blank pages. I don't know if this is the problem but it could be if there are pages of diagrams and tables.
Another thought. Have you tried to convert them with Calibre to, say epub, and see what you get. If that allows them to show the text then there is a problem with the PDF file which may be fixed by taking the epub and converting it back to PDF.
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At random a line or word will be visible on the reader, but not searchable.
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This tends to indicate there are problems with the embedding of the fonts and/or the way the reader display program reacts to the PDF version of file itself. You can check this by opening the file in a text editor and looking at the first line - it should show something like %PDF-1.3 with the numbers being the version it was created with. All of the PDFs I use are 1.3 to 1.5 - the PDF version of the edge users guide is 1.3. Again this may or may not be the cause of the problem but is worth checking.