Android：从PDF提取文本的速度非常慢

Question

In my Android project I would like to extract plain text from a pdf file. I managed to do that, tried itext as well as pdfbox library. No matter which of them is used, the process of text extraction from one page pdf file takes 2-3 minutes(!). And this not a case of a simulator, but rather a Samsung Galaxy S7 running on Android 8.0.

I tried to explore in debugger what takes so long, however with Android Studio debugger I was not able to do that, sometimes the breakpoints dont work, call stack wasnt shown when program was paused, or the program terminates out of sudden after is paused...

The code is run in a JobIntentService:

public class GetMenuService extends JobIntentService {
...

    @Override
    protected void onHandleWork(Intent intent) {
        try {
            PdfReader reader = new PdfReader(new File(getFilesDir(), MENU_FILENAME).getPath());
            int n = reader.getNumberOfPages();
            for (int i = 0; i <n ; i++) {
                parsedText   = parsedText+ PdfTextExtractor.getTextFromPage(reader, i+1).trim()+"\n"; //Extracting the content from the different pages
            }
            System.out.println(parsedText);
            reader.close();
        } catch (Exception e) {
             System.out.println(e);
        }
        ...        
    }
}

To improve the performance it is needed to know what causes the problem. Is the library so slow? Is it missing any build optimization in Android Studio? Or is it because the code is inside of JobIntentService?