It could be the reason that the source of the file caused the issue. This file wasn't a PDF initially, and was converted from some other file format to pdf (eg. an image based PDF). 

• Check the Security Settings 

1. Open the PDF
2. Click File from the top menu bar and select “Properties”
3. Switch to “Summary” from within the File Info panel
4. Check if the document was set as allow editing
5. If the editing option is allowed, it could be the file was made from scanned images

• Use OCR Converter 

1. Open the PDF
2. Tap the OCR button on the toolbar
3. Choose either way you want to perform OCR on your PDF
4. Extract the text content for further editing