300dpi TIFF images using File>Save as Other>Image>TIFF - just make sure you click on the Settings button and specify your resolution.
If you have Adobe Acrobat, you can save your document as e.g. This will give you (as long as the image data is of high enough quality) a much better result when exporting data. What you can do however is to convert your PDF document into a series of image files, then convert these image files back into the PDF format and then run OCR. You mention OCR - you cannot run OCR on a document that already has text in it, so that does not help. Low quality PDF generators (which oftentimes also translates into low cost) do not embed all the information that is necessary to extract data from a PDF document - it looks good when viewed on screen or printed, but when you try to either convert the file to a different format or copy&paste data from it, you run into the problems you describe. As JR indicated, this is a problem with your PDF file.
