There are many types of file where this can be a problem that requires an editor to fix the file, however there are also some types of file where SumatraPDF may struggle compared to other viewers.
In addition different versions of SumatraPDF can handle the files contents better than others. and there are two groups of PDF file types where there are currently open issues.
- files generated using TeX e.g. PDF LaTeX i.e. often academic papers
see Cannot search / find text in some PDFs
- files generated using OCR e.g. Tesseract
see SumatraPDF copy / paste issues with Unicode text
The underlying cause is the words are not continuous obects but strings of varying shapes.
Those two groups were better handled by version 3.1.2 so if you are using a version other than that, you can try a portable version of older 3.1.2 to see if it helps.
To give a definitive answer for a specific type of file we would need to see the file which you could post at
Issues · sumatrapdfreader/sumatrapdf · GitHub