will Validity, Generally
There are four basic conditions that must be met for a Pennsylvania will to be valid and acceptable for probate.
1. The testator must be at least 18 years of age at the time of execution.
2. The testator must be of sound mind at the time of execution.
3. The will must be in writing.
4. The will must be signed at the end.

Although having each of these conditions does not guarantee a valid will, a will cannot be valid if just one of them is absent.  Pennsylvania is the only state that does not require any subscribing witnesses, unless the testator is unable to write his or her own signature to the document.  However, there are advantages to have subscribing witnesses that must always be considered.

See: Wills From Another State; Sound Mind, Generally; Fraud, Generally; Improper Execution, Generally; Wills Without Witnesses