How am I allowed to reference open file?

import java.io.*;

public class fileManager{
public String openFile(){
try{
String line;
StringBuffer document = new StringBuffer();
File inFile = new File("transactionlist.txt");
FileReader fileReader = new FileReader(inFile);
BufferedReader bufReader = new BufferedReader(fileReader);

while (true) {
line = bufReader.readLine();

if (line == null) break;

document.append(line);
}
}
catch(IOException e) {
System.out.println("File not found");
}
return null;
}
public String returnOpen(){
return fileManager.openFile();}
}