Corporate sustainability reporting has become a necessary practice in many countries, with frameworks such as the SDGs promoting collaboration and driving action on sustainability through the application of ESG principles. This paper analyzes the non-financial sustainability reports of six leading Spanish companies over the past decade, with the aim of assessing the evolution of ESG-related content and performing sentiment analysis. A variety of methodologies were employed, including text mining, sentiment analysis, and data visualization, to process and analyze the textual data extracted from PDF reports. Results show a notable increase in the reporting of environmental and governance-related content, while social content exhibited more fluctuations. The sentiment analysis revealed a negative correlation with environmental content and a positive correlation with governance content, suggesting that environmental issues often generate more negative sentiment, while good governance practices are associated with positive sentiment. The study highlights the growing relevance of sustainability in corporate reporting, while also discussing the challenges posed by evolving regulatory frameworks and reporting requirements.