Am I personally liable for my document sharing website which may host copyright infringing material?

There are at least two separate issues in play: (1) is someone other than the person who uploaded the infringing material liable to the copyright owner? and (2) are you personally liable as the owner of the business that publishes the infringing material?

As for issue one: Owners of websites that publish material written or uploaded by others are, generally, not liable if that material infringes the intellectual property, or otherwise violates a right, owned by someone else. You should learn all you can about this rule which is codified at 47 USC section 230. There are an awful lot of cases that deal with this "safe harbor" and not all are consistent. You need an intellectual property attorney to provide you with specific guidance.

As for issue two: You most certainly need to incorporate (in some form or another) and should do so soon. Where to incorporate should not be based on your assumptions about your infringement liability risk. Manage that risk properly and there is no reason you cannot incorporate in the US. You should have your intellectual property attorney bring in a corporate attorney to provide you both with advice on business formation matters. Good luck.

There's more you can do to manage your risks. You should own this company as a corporation or an LLC to protect your personal assets. Incorporating in Panana may have some tax advantages, so you may need to consult a CPA about that.

You also need to hire a knowledgeable internet/IP lawyer review your disclaimers, privacy and other policies and liability insurance situation to see if you're taking advantage of the "safe harbor" laws that protect websites from liability from user-generated content. YOu shoudl also review your choices of business name, domain name and trademark to amke sure those are well-chosen and properly registered.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.