Texts are listed below for reference. Acts written in Greek and the Septuagint (LXX) are word for word the same but is this enough evidence to say the Ethiopian read from the LXX. Phillip probably could read both. The Ethiopian probably could read Greek, but the Ethiopian language was a semitic language similar to Hebrew. Is this a dead-end question, or is there more evidence to answer this question?
Do we know if a Gentile was allowed to own an Isaiah scroll in Hebrew? That would point to an LXX Isaiah scroll. Judaism did get to the point of not allowing the scriptures to be touched by human hands, but this probably occurred after the destruction of the Temple in 70 A.D.
Now the passage of the Scripture that he was reading was this:
“Like a sheep he was led to the slaughter
and like a lamb before its shearer is silent,
so he opens not his mouth.
In his humiliation justice was denied him.
Who can describe his generation?
For his life is taken away from the earth.”
(Acts 8:32–33, ESV)
ὡς πρόβατον ἐπὶ σφαγὴν ἤχθη
καὶ ὡς ἀμνὸς ἐναντίον τοῦ κείραντος αὐτὸν ἄφωνος,
οὕτως οὐκ ἀνοίγει τὸ στόμα αὐτοῦ.
Ἐν τῇ ταπεινώσει °[αὐτοῦ] ἡ κρίσις αὐτοῦ ἤρθη
τὴν γενεὰν αὐτοῦ τίς διηγήσεται;
ὅτι αἴρεται ἀπὸ τῆς γῆς ἡ ζωὴ αὐτοῦ.
(Acts 8:32b–33, NA28)
ὡς πρόβατον ἐπὶ σφαγὴν ἤχθη
καὶ ὡς ἀμνὸς ἐναντίον τοῦ κείροντος αὐτὸν ἄφωνος
οὕτως οὐκ ἀνοίγει τὸ στόμα αὐτοῦ.
ἐν τῇ ταπεινώσει ἡ κρίσις αὐτοῦ ἤρθη,
τὴν γενεὰν αὐτοῦ τίς διηγήσεται;
ὅτι αἴρεται ἀπὸ τῆς γῆς ἡ ζωὴ αὐτοῦ,
(Isa. 53:7–8, LXX)
כַּשֶּׂה֙ לַטֶּ֣בַח
יוּבָ֔ל וּכְרָחֵ֕ל לִפְנֵ֥י גֹזְזֶ֖יהָ נֶאֱלָ֑מָה
וְלֹ֥א יִפְתַּ֖ח פִּֽיו׃
מֵעֹ֤צֶר וּמִמִּשְׁפָּט֙ לֻקָּ֔ח
וְאֶת־דּוֹרֹ֖ו מִ֣י יְשׂוֹחֵ֑חַ
כִּ֤י נִגְזַר֙ מֵאֶ֣רֶץ חַיִּ֔ים
(Isa 53:7–8, BHS2006)
like a lamb that is led to the slaughter,
and like a sheep that before its shearers is silent,
so he opened not his mouth.
By oppression and judgment he was taken away;
and as for his generation, who considered
that he was cut off out of the land of the living,
(Isa. 53:7b–8a, ESV)
Like a sheep being led to slaughter,
Like a ewe, dumb before those who shear her,
He did not open his mouth.
By oppressive judgment he was taken away,
Who could describe his abode?
For he was cut off from the land of the living
(Isa. 53:7b–8a, JPS1985)