The essential document for applying to the UPSC Civil Services Examination is the Online Application Form, which must be filled out accurately on the official UPSC website. This form requires candidates to provide personal details, educational qualifications, and preferences for services. It's crucial to ensure that all information is correct, as discrepancies can lead to disqualification. Alongside the form, candidates must also submit necessary certificates and identification proof, adhering to the guidelines stipulated by UPSC.