Colored Artifacts while transforming PDF to NSImageRep (png) - objective-c

Question

I want to convert different PDF pages to an png. After that I iterate through all pixels to search colored pixel. The main goal is to get the colored pages of the PDF.
For the most pages it runs great. But on some pages I have colored artifacts in red (left beside a letter) and blue (right beside a letter).
This is the original from the PDF:
(source: brebook.de)
And this is the converted letter in the png:
How can I prevent this ugly artifacts. I can't use my whole idea with this colored pixel.
This is the code which is converting the single page to a png:
//Path for saving colored Pages
NSURL *savePath = [self.homePath URLByAppendingPathComponent:#"brebook_temp/"];
//PDFDocument *thePDF = [[PDFDocument alloc] initWithURL:self.filename];
NSPDFImageRep *img = [NSPDFImageRep imageRepWithContentsOfURL:self.filename];
int coloredPages = 0;
for(int i = 0; i < self.getNumberOfPages; i++){
#autoreleasepool {
//set current page label to current page
self.currentPage = i + 1;
//set current page to i
[img setCurrentPage:i];
//create a new NSImage instance
NSImage *singlePage = [NSImage new];
//set actuall page
[singlePage addRepresentation:img];
//get "old" size
NSSize oldSize = [singlePage size];
//edit the size to 72dpi
NSSize newSize;
newSize.width = oldSize.width * 150/72;
newSize.height = oldSize.height * 150/72;
//make new image
NSImage *resizedImage = [[NSImage alloc] initWithSize: NSMakeSize(newSize.width, newSize.height)];
//write into new image
[resizedImage lockFocus];
[singlePage drawInRect: NSMakeRect(0, 0, newSize.width, newSize.height) fromRect: NSMakeRect(0, 0, oldSize.width, oldSize.height) operation: NSCompositeSourceOver fraction: 1.0];
[resizedImage unlockFocus];
//Set URL for single pages
NSURL *pageFilename = [savePath URLByAppendingPathComponent: [NSString stringWithFormat:#"Seite_%d.png", i+1]];
[[NSFileManager defaultManager] createFileAtPath: [pageFilename path]
contents:[[NSBitmapImageRep imageRepWithData:[singlePage TIFFRepresentation]]
representationUsingType:NSPNGFileType properties:nil]
attributes:nil];
if([self getColoredPixelOfImageFromURL:pageFilename] > 0){
coloredPages++;
NSLog(#"SEITE %d -----------------------------------------------------------------------------------------------", i+1);
_coloredPages = coloredPages;
[self.coloredPagesList appendString:[NSString stringWithFormat:#"%d,",i+1]];
}else{
NSError *error = nil;
[[NSFileManager defaultManager] removeItemAtURL:pageFilename error:&error];
if(error){
NSLog(#"%#", error);
}
}
resizedImage = nil;
singlePage = nil;
}
}
[self.appTimer invalidate];
Thank you so much for helping!!!