Convert PDF to JPEG

I'm trying to create a process of converting base64 of pdf to base64 of jpeg image. The problem that I have is if pdf is scanned with less than 600dpi, image is not rendered correctly, it is white image without any content in it.

This is my code:

public String convertPDFtoJPEG(byte[] pdfData) {
try {
PDDocument document = PDDocument.load(pdfData);
PDFRenderer renderer = new PDFRenderer(document);
for (int pageIndex = 0; pageIndex < document.getNumberOfPages(); pageIndex++) {
BufferedImage image = renderer.renderImageWithDPI(pageIndex, 300);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ImageIO.write(image, "JPEG", baos);
baos.flush();
String base64JPEG = Base64.getEncoder().encodeToString(baos.toByteArray());
baos.close();
}
document.close();
} catch (IOException e) {
e.printStackTrace();
}
return base64JPEG;
}
public byte[] readBase64FromFile(String filePath) {
File file = new File(filePath);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
String line;
while ((line = reader.readLine()) != null) {
baos.write(line.getBytes());
}
} catch (IOException e) {
e.printStackTrace();
} 
return Base64.getDecoder().decode(baos.toByteArray());
}

This is how I'm calling this methods:

String filepath = "path:\\to\\file.txt"
byte[] pdfData = readBase64FromFile(filePath);
convertPDFtoJPEG(pdfData);

In file.txt is base64 of pdf.

As discussed in the comments, make sure you have the dependencies for JPX and JBIG2 image formats:

<dependency>
<groupId>com.github.jai-imageio</groupId>
<artifactId>jai-imageio-jpeg2000</artifactId>
<version>1.4.0</version>
</dependency>
<dependency>
<groupId>com.github.jai-imageio</groupId>
<artifactId>jai-imageio-core</artifactId>
<version>1.4.0</version>
</dependency>

I also recommend using the twelvemonkeys library for TIFF and JPEG.

There are several issues with your code:

1. You are not decoding the base64 data correctly. You read the file line by line and convert (each line) to bytes. Then at the end you pass the whole thing to the Base64 decoder converting it back to string. You should change the method to:

   public static byte[] readBase64FromFile(String filePath) {
   File file = new File(filePath);
   StringBuilder base64StringBuilder = new StringBuilder();
   try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
   String line;
   while ((line = reader.readLine()) != null) {
   base64StringBuilder.append(line);
   }
   } catch (IOException e) {
   e.printStackTrace();
   }
   return Base64.getDecoder().decode(base64StringBuilder.toString());
   }
   

   Or read the whole thing as a binary:

   public static byte[] readBinaryFromFile(String filePath) {
   Path path2Pdf = Paths.get(filePath);
   byte[] pdf = new byte[]{};
   try {
   pdf = Files.readAllBytes(path2Pdf);
   } catch (IOException e) {
   e.printStackTrace();
   }
   return Base64.getDecoder().decode(pdf);
   }
   

2. Your convertPDFtoJPEG method doesn't make sense. You are reading the whole file but only return the last page (which might even be a white page). You should return a list of base64 jpeg pages:

   public static List<String> convertPDFtoJPEG(byte[] pdfData) {
   List<String> listOfBase64Images = new ArrayList<>();
   try (PDDocument document = PDDocument.load(pdfData)){
   PDFRenderer renderer = new PDFRenderer(document);
   for (int pageIndex = 0; pageIndex < document.getNumberOfPages(); pageIndex++) {
   BufferedImage image = renderer.renderImageWithDPI(pageIndex, 300);
   ByteArrayOutputStream baos = new ByteArrayOutputStream();
   ImageIO.write(image, "JPEG", baos);
   listOfBase64Images.add(Base64.getEncoder().encodeToString(baos.toByteArray()));
   }
   } 
   catch (IOException e) {
   e.printStackTrace();
   }
   return listOfBase64Images;
   

   }

And fyi: calling close() and flush() on a ByteArrayOutputStream doesn't do anything.