A reviewer can only log in to the portal (for example, with an auto login token) and see the correct abstracts when the Presentations module is configured correctly.
The reviewer must have:
An online account (not just a contact record)
A paper to be reviewed (either because they've been directly allocated to review on the presentation item, or the paper was allocated, via the Abstract Workflow App, to a review panel they're part of)
In other words, the reviewer must have a pending review allocated to them, visible on their Attendee Record.
For a presentation submission to be visible to the reviewer:
The presentation needs to have a presentation type allocated.
The paper status and type needs to match the visibility settings on the Reviewer Portal (and those settings have to be enabled/set up in the Reviewer Portal)
Other things to check:
Missing or incorrectly entered weighting scores for review criteria (in the Reviewer Portal setup) can block abstracts from being displayed to reviewers.
Search this site for the full guide to setting up abstract reviews and presentation management.