> Why is Portable Document Format not always portable?
why is it that preview and third–party apps can’t open some pdf files properly, but adobe acrobat is able to?
Although PDF is an open standard, it has a long and complex history. It encompasses several standards, including PDF/A for archival use, PDF/X for pre–press, and PDF/UA for accessibility. The Quartz 2D PDF engine in macOS can read each variant, but doesn’t cater for their special features, which are left up to apps to handle.
Problems become most apparent when a document complies with the PDF/X–4 standard, which is widely used in pre–press, because it caters for process and spot color, for example. Preview and apps that rely on the Quartz 2D engine are not able to render those files correctly, shown as “X” marks by the engine’s test suite, which disrupts color graphics.
Adobe Acrobat, even the free Reader DC version, makes use of Adobe’s own PDF engine internally. So with Reader, essentially you get the same rendering as in its expensive Pro version, but with limited features beyond that. Adobe’s engine is compliant with all the major PDF variants, including the PDF/X family, so should render documents flawlessly.
Writing a PDF engine to support all these standards is a fearsome task, and a huge investment for a small market. Adobe’s had over 25 years to get it right, and wrote the predecessors of the current PDF standards.