This article describes compatibility with 3rd-Party apps and potential issues when using them.
PDF Viewer writes annotations in accordance to the standardized PDF specification. Any compliant viewer (like Adobe Acrobat) will be able to display and edit such annotations and forms. Using standard compliant software, annotations can be created e.g. using PDF Viewer for iOS, edited in Adobe Acrobat for Windows and later displayed and edited/deleted on PDF Viewer for Android without any problems.
Apple macOS High Sierra/Sierra (Safari/Preview/QuickLook)
Apple uses a custom PDF renderer which is quite capable of viewing PDFs, however it is known to freeze/crash or not show content on several files. Most documents render correctly or well enough to be usable. Annotations are a different story. While most annotations render correctly, there are issues related to rotated documents, form editing, stamps, and other features.
Apple improved PDF support in iOS 11, which extends annotation support and is able to display most commonly used annotations (highlight, ink, free text). There are still significant bugs and issues where annotations are not displayed, are mirrored or positioned incorrectly. iOS 11 also allows annotating PDF documents, however, this component is not accessible to apps. These annotations are partly standard compliant, but include Apple-specific additions. That means displaying them in other viewers works correctly, however, editing will remove any Apple-specific additions, which might break them.
Popular 3rd-Party Apps on iOS with Annotation Support
Popular 3rd-party apps on iOS are Adobe Acrobat Reader, PDF Expert, iAnnotate, GoodReader, Tiny PDF, and PDF Box. These all support standardized annotations, which should be visible when sharing between them.
We can't fix issues for other apps. If you encounter an issue with an app that doesn't handle something properly, please report those issues to the responsible company. Thanks!
You can read more about 3rd-party compatibility in the guides of the underlying PDF framework of PDF Viewer - PSPDFKit.