Filtering kinda "half-toned" image for OCR processing
What you really require is probably some morphological operation like dilation followed by erosion. This is called as Closing operation. May be in your case- just dilation itself might be good. 

There was a similar question asked previously - which can help with other aspects. 

monochrome (1-bit black-and-white) image conversion

How do I reconstruct text from an image using only morphological operations?