Where can I learn more about PDF file formats?

PDF (Portable Document Format) is an open standard. Adobe published the format in 1993 and handed it to ISO in 2008: ISO 32000-1 is PDF 1.7 and ISO 32000-2 is PDF 2.0, which is what current software produces. Several specialised profiles sit on top of it: PDF/A (ISO 19005) for long-term archiving, PDF/X (ISO 15930) for print production, PDF/UA (ISO 14289) for accessibility, and PDF/E for engineering drawings.

Good places to read on:

• PDF Association - the industry body that maintains the standards; it publishes the ISO 32000-2 text free of charge and explains the PDF/A, PDF/X and PDF/UA profiles.
• ISO 32000-1 (PDF 1.7) reference - the full specification, hosted by Adobe.
• Wikipedia: PDF - history, structure and a comparison of the variants.
• veraPDF - the open-source validator used to check PDF/A conformance.
• Ghostscript documentation - the interpreter behind many PDF tools; its "Screen", "Ebook", "Printer" and "Prepress" quality settings are the presets you see on Compress PDF and PS to PDF.

On XConvert you can create PDFs from documents and images (PDF converter), merge, split, compress, rotate and watermark them, convert them to JPG, PNG, DOCX or EPUB, and go back to PostScript. All document tools are listed under Document tools.