I have seen the product SHOUT used to successfully remove certain stains and yellowing from certain pennants. The product should be sprayed onto another cloth worked in a bit then applied via the cloth to the pennant. I have seen this method work wonders in certain situations. However, the biggest problem you face with this method is that you may successfully remove the spot or yellowing but are then left with a really clean spot on a pennant which probably has patina throughout.  This is a decent method to try with a newer pennant with a coffee stain or something similar.   You will face this same problem regardless of the cleaner you use.  I have also seen this method used as a sort of bath with a two parts to one mixture of water to cleaning solution.  Be sure not to create a very soapy solution as you run the risk of a soapy residue once the pennant dries. The entire pennant up to the spine (Not including the spine) is then dipped or bathed in the solution for as long as it takes to get the desired effect.  By dipping the pennant you will start the agitation process and hopefully expedite the cleaning process.  You must be aware that there is the potential of bleeding on the pennant surface itself and into the spine. Now I have seen collectors remove the spine entirely when using this method as well. Once the pennant has been cleaned to your satisfaction, remove it from the cleaning solution. You will also want to rinse the item in a plain water bath after cleaning.  Do not rinse under a faucet and do not wring the pennant out!!!  I have also seen Woolite or a soft detergent like Surf utilized which is good for removing mold and fungus.