I'll try to answer your question, sorry if it doesn't satisfy you. My answer will be about the immigration to America generally, since there's a lack of literature regarding Brazil specifically

What are the reasons for this immigration

A lot of reasons, but mostly to improve their economic conditions. During the last half of the 19th century, changes in the economic and ethnocultural structure of Ottoman society coupled with the industrialization of North America and the rise of large agricultural enterprises in South America incite people in the Ottoman Empire to migrate, just like everywhere else. That era marked the pouring of millions of Crimean, Balkan, and Caucasian refugees to Ottoman lands which was later settled and given lands to cultivate. While this resulted in an agricultural revolution and overall increase of revenue in Ottoman agricultural economy, not all people benefited from this. The coastal areas with relatively rich agricultural lands and ports developed rapidly, but the interior due to lack of transportation and other causes, did not immediately feel the benefit of the socioeconomic boom occurring along the privileged coasts. In some areas of Anatolia and Syria the immigrant Bosnians, Circassians, Cretans, and Turks were prospering, while some Syrians, tied to their old land and unable to accept innovative methods remained economically stagnant and socially and culturally conservative. On the other hand, there were also immigrants who, unable to adapt to their new homes, did not prosper. In addition, the rapid occupational transformation of the cities left many traditional craftsmen and professionals unemployed because their skills had become obsolete in the new society. Census by the Ottoman government between 1880 and 1900 shows that practitioners of old crafts and professions are dwindling in number, but on the other hand many new professions like working in transportation, banking, insurance and the like are expanding. The desire to escape from a condition of poverty or to remedy one's deteriorating economic situation by moving to a place that offered the possibility of bettering oneself was a powerful one. The early Syrian migrants to the Americas were mainly from the lower socioeconomic classes, some even sold almost all of their belongings simply to pay for their passage. If those emigrants were successful, they will usually return home to buy land, put up large houses, and become a rural upper class who then later tell tales of the wealth of the Americas and entice more people to emigrate drawn by the prospect of increasing their wealth. Between 1820 and 1920, around 1,2 million people from the Ottoman lands emigrated to the New World

Were a significant portion of these immigrants Muslim, or were they all Christian? If there was a significant Muslim portion, I assumed they mostly converted, as their descendants are not (AFAIK) Muslim. If so, were they pressured directly or indirectly to convert when they arrived?

From 1,2 million number i said earlier, approximately 15% or roughly 200,000 were Muslims. Most of them converted, mostly because of pressure towards Muslims at the time which i'll focus on the United States. In the US, stories about oppression, injustice, and maltreatment at the hands of the Ottoman government and their Muslim citizens not supported by the facts were published in the press of the United States and elsewhere, often backed up by reports from missionaries or local priests who were prone to interpret any act of authority on the part of the Ottoman government as "oppression" and any demand for the payment of legitimate taxes as "extortion". On the turn of the 20th century, immigrant intellectuals fanned this further by publishing journals and write books espousing various brands of nationalism and to consider themselves the political spokesmen for those back home. One of the techniques for arousing Christian sympathy was to claim to have been driven out of Jerusalem by fanatic Muslims, even though the writer's original home had been elsewhere in Syria or Anatolia and far from Jerusalem.

With anti-Muslim sentiments in the United States, many Muslim (or Turkish) emigrants decided to identify as "Albanian", "Bulgarian", "Serbian", and "Syrian" Christian to not arouse the interest of Ottoman officials or jeopardize their entry into their new countries which they soon discovered preaching religious equality in public but was intolerant of it in private. Many took on Christian names, and it is certain that a large number actually converted to Christianity (or their children did). Some Turkish Muslim children also were converted through being adopted by Bulgarian, Serbian, or Bosnian families, after their parents had been murdered by nationalist terrorist bands or slaughtered by Bulgarians and Serbians in Macedonia during the Balkan war, and these Muslim children entered the United States under European Christian headings when their adoptive families immigrate. There were also other inducements to conversion other than the simple desire to blend in with the rest of society, these were quite attractive to the point that in a number of cases Christian immigrants claimed that they were Muslims and expected to be induced to "convert" at some gain for themselves

Some of the early Muslim communities survive in debilitated form in various cities of North and South America, but most seem to have disappeared as their membership dwindled through conversion, death, and especially, return to the home country. The lack of Muslim leadership to provide support and guidance and the scarcity of Muslim women among the immigrant accounted in large part for the decline of these communities as they never became strong enough to enable their members to resist conversion pressures or to make the New World culturally attractive enough to be considered a permanent home by those who did not care to convert

Sources:

The Turks in America and The Ottoman Emigration to America 1860-1914 by Kemal H. Karpat