So, this, at least in the region today is a very politically charged question, and the wrong answer could seriously get you beaten in places. And that answer varies whether you are in Skopje, Sofia, Belgrade or Athens.

From historical perspective Bulgaria has some serious claims over the region - part of the Bulgarian kingdom, and even a capital of it (Ohrid) was located there. But so do the Serbs, Greeks, Albanians and so on. To make things even better - during the time of the Ottoman Empire there was a lot of moving around, languages and cultures spreading, so basically every region was quite mixed, Macedonia was not an exception.

Now starting sometime in the 18-19th century things like national identities started forming, somewhat under the influence of Western philosophy and ideas nonetheless. This (among other factors) was the cause rebellions and wars, that eventually lead to the forming of modern days Greece, Bulgaria, Macedonia, Serbia, etc. Now all of these had the idea of the national state - a state encompassing all the lands where their people lived. So for Bulgaria that would be half of modern Serbia, modern Macedonia, big pars of modern Romania, a lot of Northern Greece, Even parts of Turkey. You can imagine that the other countries had a similar ideas.

Macedonia is kind of the crux of this conflict - it is a very ethnically mixed place, which each of the three neighbours claims as integral to their lands and nation. Before World War I there where two Balkan wars (pretty big conflicts, setting up the stage for WWI and even WWII for the region), that had a big focus on Macedonia. During these times all the countries engaged in some kind of repressions, to silence people from other minorities. There was a lot of movement of people around (i.e. there was a big refugee movement of macedonian Bulgarians after WWI - my great-grandfather was Macedonian). But this applies for other ethnicities as well.

Today, I would say that Macedonians are their own people, that have undergone similar identity-forming processes as other places in the region. In a sense they are the same as Bulgarians, just as they are the same as Serbians and the same as Greeks and Albanians. But just as everything else on the Balkans, it is a clusterfuck, and the only thing left is to let people identify themselves however they want.

I have read plenty of texts from the 1900s and sources. Texts from Léon Lamouche: http://data.bnf.fr/12932391/leon_lamouche/ seem to point out that a large part of people living in today's Macedonia used to identify as Bulgarians.

This is a sort of linguistic sleight of hand--identity categories in the Balkans in the early 20th century don't line up neatly with the categories we have today. Mazower (2000) discusses the fight between early Greek and Bulgarian nationalists over the identity of the inhabitants of modern-day Macedonian, relating the following tale from a "Greek activist"^1:

On my arrival in Salonika the idea of Greek peasants and the people on the actual difference between the Greek Orthodox church and the Bulgarian schismatics was rather shaky. I realized this because whenever I asked them, what they were--Romaioi [i.e., Greeks], or Voulgaroi [Bulgarians], they stared at me incomprehendingly. Asking each other what my words meant, crossing themselves, they would answer me naïvely: 'Well, we're Christians--what do you mean, Romaioi or Voulgaroi?'

He relates a similar story about a Bulgarian (related via an Englishman), who

tried to explain to them that by being forced to listen to the church service in Greek rather than in their own Slavic tongue, they were being exploited by an anti-Bulgarian clergy. "But they bolted raw cabbage and washed it down with mastic and only said that it did not matter; many of them spoke Greek. The priest took a suck at the bottle and was of the same opinion. He spoke a local Slavic dialect himself for ordinary purposes, but he had learned all the services in Greek. It was a good service and what did it matter?"

The take-away here is that there weren't really categories of "Bulgarian" or "Macedonian" or even "Greek" for a lot of inhabitants of the Balkans, even in the early 20th century. People spoke multiple languages as a matter of course--in the countryside, a variety of Slavic, Albanian, or Romance (depending), in the towns Greek (and perhaps also Judeo-Spanish), in the mountains Vlach or Albanian, and if someone was a Rrom, they spoke Rromani and probably a few other languages besides. When you look at late 19th/early 20th century ethnographies of the Balkan countryside, before nationalism really became a thing, is that there were two main categories of identity: Turk (i.e. Muslim) vs Christian. Whether you spoke Turkish or Albanian or Greek or Slavic or Romance at home, if you were Muslim you were a Turk (although this was a derogatory term to some extent), and if you were Christian you were a either Bulgarian or Greek--depending on location--which is to say, a Christian. So, while 19th/20th century peasants in Macedonia might have used words like Bălgar to describe their identity or were described as such in censuses (though many did not recognize the term, as we saw above), that doesn't mean we can equate their identity with Bulgarian identity after the Balkan Wars.

And, it's worth noting, from a linguistic standpoint Macedonian dialects have been distinguished from Bulgarian dialects for some time--they're on a continuum together, of course, and there are a lot of isoglosses that fan out across Macedonia, but they are different, and Bulgarian shouldn't be accorded special status because they decided on a standard language before Macedonians did. People have been writing about Macedonian varieties of Slavic since the 1910s--Macedonian dialects are distinguished from Bulgarian ones in quite real ways: the stops /c ɟ/ <ќ ѓ>, for instance. A very old difference, dating to two of our very oldest Slavic texts, the Codices Zographensis and Marianus, is that in Macedonian (and nearby dialects in Bulgaria) the Common Slavic vowel *ъ (pronounced something like oo in English book) developed into /o/ (as in son 'dream'), and the vowel *ь (something like i in English bit) developed into /e/ as in temen 'dark', while in more easterly Slavic dialects (including what became Standard Bulgarian) both developed into /ɤ/ (Bulgarian săn and tămen). Standard Macedonian has mandatory clitic doubling, while it's only mandatory in Standard Bulgarian in certain contexts related to definiteness (here I'm simplifying a complex literature on Bulgarian, but forgive me--I'm not a syntax guy). Again, this difference is reflected in texts that date back to the tenth century--it's not some modern innovation by nefarious nation-building Yugoslavs or anything.

As a final note, I'll say that I'm not familiar with instances of the Yugoslav state repressing self-identified Bulgarians in the Republic of Macedonia--which isn't to say that it didn't happen, but I've not heard such stories. I can say that Bulgaria did a lot to undermine the legitimacy of Macedonian as an independent language, going so far as to at least once refuse to give a conference-related visa to the linguist Horace Lunt, who published the first grammar of the Macedonian language in 1952. While he was on a fieldwork trip in Macedonia to gather material for that grammar, in fact, he was given a smuggled Bulgarian newspaper that proclaimed "local talent was insufficient and the hapless Yugoslavs had been forced to import a spy to create the language for them. Takiva horaslantovtsi nam ne triabvat!" ("We don't need such Horace Luntites!").