Can I convert only page 1 without processing the rest?

Yes: extract that page first with the Split tool and convert the resulting file.

What resolution suits a web thumbnail?

96 or 150 dpi is plenty. 300 dpi only if it will be printed or enlarged.

Get a thumbnail or cover from a PDF's first page, without uploading the document.

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For a thumbnail on a website, a report cover or a catalogue preview, the first page is almost always enough.

Convert the whole PDF and keep the first image, or extract page 1 first with "Extract a single page" and convert just that: faster on long documents.

PNG is preselected because it keeps text sharp; switch to JPG if the cover is a photograph and you want it lighter.

Can I convert only page 1 without processing the rest?: Yes: extract that page first with the Split tool and convert the resulting file.
What resolution suits a web thumbnail?: 96 or 150 dpi is plenty. 300 dpi only if it will be printed or enlarged.