Postal Service Financial Improvement Act of 2017 This bill requires the Department of the Treasury to: (1) invest a specified percentage of the Postal Service Retiree Health Benefits Fund, using one or more qualified professional asset managers, in index funds modeled after those established for Thrift Savings Fund investments; and (2) ensure that the investment replicates the performance of the longest-term target date asset allocation investment fund established by the Federal Retirement Thrift Investment Board. The bill establishes the Postal Service Retiree Health Benefits Fund Investment Committee, which Treasury shall consult regarding such investments. The "specified percentage" to be invested is defined as 25% of the currently available portions of the fund as are not immediately required for payments from the fund, except that the committee may specify a higher percentage, not to exceed 30%, not earlier than five years after this bill's enactment and as appropriate thereafter. Treasury shall annually engage an independent qualified public accountant to audit the financial statements of such investments and shall submit an annual management report that includes: statements of financial position, operations, and cash flows; a statement on internal accounting and administrative control systems; the report resulting from the audit; and any other comments and information necessary to inform Congress about the operations and financial condition of the investments.