Conversational and detailed
Pamela consists almost entirely of letters, which means it's—drumroll—an epistolary novel. This was pretty much the way people wrote novels up through the end of the eighteenth century, and it was designed to give their works a kind of first-person reporting feel.
(Quick Brain Snack: People weren't sure at first whether it was really okay to be reading novels—think the way people felt about TV back in the 1980s and 1990s—so a huge majority of early novels spent a lot of time insisting that their stories were actually factual. It's okay to sit in front of the boob tube if you're watching a PBS documentary, you know? Not that people necessarily believed all the True Story hokum, although some probably did.)
In any case, because Pamela's "voice" dominates the novel, its style is pretty much hers. She is conversational and straightforward while describing the events that have transpired, though she also infuses her tale with plenty of detail, emotion, and moral commentary.
Some of her most entertaining and vivid narration occurs when she is describing someone she doesn't like—for example, Mrs. Jewkes:
Now I will give you a Picture of this Wretch! She is a broad, squat, pursy, fat Thing, quite ugly, if anything God made can be ugly; about forty Years old. She has a huge Hand, and an Arm as thick as my Waist, I believe. Her Nose is flat and crooked, and her Brows grow over her Eyes; a dead, spiteful, grey, goggling Eye, to be sure, she has. And her Face is flat and broad; and as to Colour, looks like as if it had been pickled a Month in Salt-petre: I dare say she drinks!—She has a hoarse man-like Voice, and is as thick as she's long; and yet looks so deadly strong, that I am afraid she would dash me at her Foot in an Instant, if I was to vex her.—So that with a Heart more ugly than her Face, she frightens me sadly; and I am undone, to be sure, if God does not protect me; for she is very, very wicked—indeed she is. (38.6)
Pamela may be a sweetheart, but she knows how to grab our interest. She's conversational and vivid rather than stuffy and removed. We dig it.