Dear Ms. Gianvincenzo,
Thank you for posting your request on History Hub!
We suggest that you review NARA’s website at Passport Applications for background information about the passport process. To have received a U.S. passport in 1928, a person was required to be an American citizen. Although the passport number likely will not help with the search, knowing where your ancestor resided during his time in the U.S. can be extremely useful. If you know the city and state where he lived, we suggest you that you search for the National Archives facility that holds naturalization records from the Federal court(s) for that locality to determine if any records exist at National Archives Court Records.
We also suggest that you search the Population Schedules for the 1910 Census for Indiana, PA as a first start to narrow down the time frame in which his naturalization could have occurred. Please look for “Na” in the citizenship column which will indicate whether he had received US citizenship by that time period. The 1910 Census has been digitized and is available for free on the NARA catalog link above.
Since Indiana County, PA has not been added to the Catalog yet for the 1920 and 1930 Population Schedules, digitized census records may be found for 1920 and 1930 using Ancestry. There may be a fee associated with using Ancestry but you may access the digital images at a NARA facility for free using a NARA PC.
We hope this is helpful. Best of luck with your family research!
[Some information provided by Elizabeth Burnes]