From Tom Moldvay's Foreword to the Basic Rulebook (page B2, and dated 3 December 1980):
I was busy rescuing the captured maiden when the dragon showed up. Fifty feet of scaled terror glared down at us with smoldering red eyes. Tendrils of smoke drifted out from between fangs larger than daggers. The dragon blocked the only exit from the cave. . .
I unwrapped the sword which the mysterious cleric had given me. The sword was golden-tinted steel. Its hilt was set with a rainbow collection of precious gems. I shoulted my battle cry and charged.
My charge caught the dragon by surprise. Its titanic jaws snapped shut just inches from my face. I swung the golden sword with both arms. The swordblade bit into the dragon's neck and continued through to the other side. With an earth-shaking crash, the dragon dropped dead at my feet. The magic sword had saved my life and ended the reign of the dragon-tyrant. The countryside was freed and I could return as a hero.
From the Introduction to the same book (page B3):
In the D&D rules, individuals play the role of characters in a fantasy world wher magic is real and heroes venture out on dangerous quests in search of fame and fortune.
From the Introduction to Gary Gygax's Player's Handbook (p 7):
ADVANCED DUNGEONS & DRAGONS is a world. Of course, this world is not complete. It needs organizers and adventurers to order and explore it. It neds you! . . . Into this world of weird monsters, strange peoples, multitudinous states, and fabulous treasures of precious items and powerful magic stride fearless adventurers - you and your fellow players.
Under the heading "The Game" in the same book (p 7):
[O]ne player must serve as the Dungeon Master, the shaper of the fantasy milieu, the "world" in which all action will take place. The other participants become adventurers by creating characters to explore the fantastic world and face all of its challenges - monsters, magic, and unamed menaces.
Moldvay's Foreword is the most evocative of these passages, but all seem to me to very strongly imply that the PCs are protagonists in a fantasy adventure.
The PCs venture out on dangerous quests in search of fame and fortune, striding through the world created by the GM (Gygax's description of this is reminiscent of Conan as described in the Nemedian Chronicles), exploring that world and facing all of its challenges.