The triangle $ABC$ has a right angle at $C.$ The point $P$ is located on segment $AC$ such that triangles $PBA$ and $PBC$ have congruent inscribed circles. Express the length $x = PC$ in terms of $a = BC, b = CA$ and $c = AB.$
1. **Given Information and Setup:**
   - We have a right triangle \( \triangle ABC \) with \( \angle C = 90^\circ \).
   - The sides are denoted as \( a = BC \), \( b = CA \), and \( c = AB \).
   - Point \( P \) is on segment \( AC \) such that the inscribed circles of \( \triangle PBA \) and \( \triangle PBC \) are congruent.
   - We need to express \( x = PC \) in terms of \( a \), \( b \), and \( c \).

2. **Inradius and Altitude:**
   - The altitude from \( B \) to \( AC \) is \( h = a \) because \( \angle C = 90^\circ \).
   - The inradius \( r \) of \( \triangle ABC \) is given by:
     \[
     r = \frac{\text{Area of } \triangle ABC}{\text{semiperimeter of } \triangle ABC} = \frac{\frac{1}{2}ab}{\frac{a + b + c}{2}} = \frac{ab}{a + b + c}
     \]

3. **Congruent Incircles Condition:**
   - Let \( u \) be the inradius of both \( \triangle PBA \) and \( \triangle PBC \).
   - Using the formula for the sum of the reciprocals of the inradii:
     \[
     \frac{2}{h} = \frac{1}{u} + \frac{1}{u} - \frac{r}{u^2}
     \]
     Since \( h = a \) and \( u = v \), this simplifies to:
     \[
     \frac{2}{a} = \frac{2}{u} - \frac{r}{u^2}
     \]

4. **Solving for \( u \):**
   - Substitute \( r = \frac{ab}{a + b + c} \) into the equation:
     \[
     \frac{2}{a} = \frac{2}{u} - \frac{\frac{ab}{a + b + c}}{u^2}
     \]
   - Multiply through by \( u^2 \):
     \[
     2u^2 = 2au - \frac{abu}{a + b + c}
     \]
   - Rearrange to form a quadratic equation:
     \[
     2u^2 - 2au + \frac{abu}{a + b + c} = 0
     \]
   - Factor out \( u \):
     \[
     u \left(2u - 2a + \frac{ab}{a + b + c}\right) = 0
     \]
   - Solve for \( u \):
     \[
     2u - 2a + \frac{ab}{a + b + c} = 0
     \]
     \[
     2u = 2a - \frac{ab}{a + b + c}
     \]
     \[
     u = a - \frac{ab}{2(a + b + c)}
     \]

5. **Simplifying \( u \):**
   - Using the Pythagorean theorem \( a^2 + b^2 = c^2 \):
     \[
     u = \frac{a}{2} \left(1 - \sqrt{\frac{2bc - 2b^2}{2ab}}\right)
     \]
     \[
     u = \frac{a}{2} \left(1 - \sqrt{\frac{c^2 - b^2}{a^2}}\right)
     \]
     \[
     u = \frac{a}{2} \left(1 - \sqrt{\frac{a^2 - (c - b)^2}{(a + b)^2 - c^2}}\right)
     \]
     \[
     u = \frac{a}{2} \left(1 - \sqrt{\frac{2bc - 2b^2}{2ab}}\right)
     \]
     \[
     u = \frac{1}{2} \left(a - \sqrt{a(c - b)}\right)
     \]

6. **Conclusion:**
   - The length \( x = PC \) is given by:
     \[
     x = \frac{1}{2} \left(a - \sqrt{a(c - b)}\right)
     \]

The final answer is \(\boxed{\frac{1}{2} \left(a - \sqrt{a(c - b)}\right)}\)