Also see Parts 1, 2, and 3.
As regards taxation, assessment according to ability to pay is fundamental to a just and equitable system.
-- Mother and Teacher, #132
These duties call not only for individual charitable giving but also for a more systematic approach by businesses, labor unions, and the many other groups that shape economic life -- as well as government. The concentration of privilege that exists today results far more from institutional relationships distribute power and wealth inequitably than from differences in talent or lack of desire to work. These institutional patterns must be examined and revised if we are to meet the demands of basic justice. For example, a system of taxation based on assessment according to ability to pay  is a prime necessity for the fulfillment of these social obligations.
-- Economic Justice for All, #76
More specifically, it is the responsibility of all citizens, acting through their government, to assist and empower the poor, the disadvantaged, the handicapped, and the unemployed...Government may levy the taxes necessary to meet these responsibilities, and citizens have a moral obligation to pay those taxes.
-- Economic Justice for All, #123
Businesses have a right to an institutional framework that does not penalize enterprises that act responsibly. Governments must provide regulations and a system of taxation which encourage firms to preserve the environment, employ disadvantaged workers, and create jobs in depressed areas. Managers and stockholders should not be torn between their responsibilities to their organizations and their responsibilities toward society as a whole.
-- Economic Justice for All, #118
Government officials, it is your concern to mobilize your peoples to form a more effective world solidarity, and above all to make them accept the necessary taxes on their luxuries and their wasteful expenditures, in order to bring about development and to save the peace.
-- On the Development of Peoples, #84
The struggle against destitution, though urgent and necessary, is not enough. It is a question, rather, of building a world where every man, no matter what his race, religion or nationality, can live a fully human life, freed from servitude imposed on him by other men or by natural forces over which he has not sufficient control; a world where freedom is not an empty word and where the poor man Lazarus can sit down at the same table with the rich man. This demands great generosity, much sacrifice and unceasing effort on the part of the rich man.
Let each one examine his conscience, a conscience that conveys a new message for our times. Is he prepared to support out of his own pocket works and undertakings organized in favor of the most destitute?
Is he ready to pay higher taxes so that the public authorities can intensify their efforts in favor of development? Is he ready to pay a higher price for imported goods so that the producer may be more justly rewarded? Or to leave his country, if necessary and if he is young, in order to assist in this development of the young nations?
-- On the Development of Peoples, #47