Please tell us what software you're using.
If if you're using Photoshop, consider using the Image Processor from within Bridge:
Are you using Photoshop, Photoshop Elements, Lightroom, Bridge...?
Using the adobe acrobat pro
That is not a good application to use for that purpose. You need a graphic image editor.
There are many of the available online, for free or not. Adobe has Photoshop, but that's really an overkill for such a task.
If you are on a Mac, you can use the Preview application (which comes with Mac OS X) to convert from one image format to a different one.