From the Guidelines

Tadalafil has no established benefit for patients with heart failure.

The provided evidence does not support the use of Tadalafil or other phosphodiesterase-5 inhibitors, such as sildenafil, for the treatment of heart failure with preserved ejection fraction (HFpEF) 1.
The RELAX trial, which investigated the effects of sildenafil on exercise tolerance in patients with HFpEF, found no improvement in oxygen consumption or exercise tolerance 1.
The NEAT-HFpEF trial also found no beneficial effects of isosorbide mononitrate on activity levels, quality of life, exercise tolerance, or NT-proBNP levels in patients with HFpEF 1.
As a result, routine use of nitrates or phosphodiesterase-5 inhibitors, including Tadalafil, is not recommended for patients with HFpEF unless they have symptomatic coronary artery disease (CAD) 1.
In patients with HFpEF and symptomatic CAD, nitrates may provide symptomatic relief, but this is not directly related to the use of Tadalafil or its effects on heart failure.

From the FDA Drug Label

The FDA drug label states that tadalafil is not recommended for patients with New York Heart Association Class 2 or greater heart failure in the last 6 months 2.
Tadalafil has mild systemic vasodilatory properties that may result in transient decreases in blood pressure, which could be a concern for patients with underlying cardiovascular disease, including heart failure 2.
No direct information is available on the effect of tadalafil on heart failure, but it is recommended to use caution when prescribing tadalafil to patients with cardiovascular disease, including heart failure 2. The FDA drug label does not answer the question.

Tadalafil, a phosphodiesterase-5 (PDE-5) inhibitor, has been shown to have cardiovascular-protective effects in patients with heart failure 3.
Studies have demonstrated that tadalafil can improve hemodynamic indexes, elevate endothelial cell-derived nitric oxide (NO) levels, and activate protein kinase A, which can be beneficial for patients with heart failure 3, 4.
In animal models, tadalafil has been shown to prevent acute heart failure with reduced ejection fraction, reduce infarct size, and attenuate cardiac hypertrophy and pulmonary edema 4.
However, a randomized controlled phase 3 study found that tadalafil did not improve outcomes in patients with heart failure with preserved ejection fraction and combined postcapillary and precapillary pulmonary hypertension, and may even have potential safety concerns 5.
Other studies have found that tadalafil can improve left ventricular diastolic function in patients with resistant hypertension, independently of blood pressure reduction 6.
Tadalafil has also been shown to prevent fibrotic remodeling and improve left ventricular function in animal models of heart failure, although its effects may be different from those of other drugs, such as saxagliptin 7.

Tadalafil's cardiovascular-protective effects are thought to be mediated by its inhibition of PDE-5, which increases cyclic guanosine monophosphate (cGMP) levels and activates protein kinase G 3, 4.
Increased cGMP levels can lead to vasodilation, reduced cardiac hypertrophy, and improved cardiac function 4, 6.
Tadalafil may also have anti-fibrotic effects, as it has been shown to reduce collagen deposition in animal models of heart failure 7.

Tadalafil may be a therapeutic option for patients with heart failure, particularly those with reduced ejection fraction or diastolic dysfunction 3, 4, 6.
However, its use in patients with heart failure with preserved ejection fraction and combined postcapillary and precapillary pulmonary hypertension should be approached with caution, due to potential safety concerns 5.
Further studies are needed to fully understand the effects of tadalafil on heart failure and to determine its potential role in the treatment of this condition.