You want to leave a sticky note. The comment tool is greyed out. Or it's available but clicking the page does nothing.
Annotation permissions are off. Easy to work around.
What's actually going wrong
PDFs can disable annotation as a permission setting. This is independent of editing and printing — the file might allow viewing and even editing but block comments and notes. Some viewers grey out the relevant tools when this restriction is set.
Less common: the PDF is corrupted in a way that breaks the annotation layer. Rare.
The quick fix
Open the PDF in annotate PDF on Flint. Flint flattens annotations into the page when you save — which works on PDFs that block traditional annotation layers because the flattened result is just page content, not annotation metadata.
If the restriction is permission-based and you have the owner password, unlock the PDF first, then annotate in any tool.
If that didn't work
If annotations need to survive editing by the next person (rather than being baked in), the restrictions are exactly what they block. Request a comments-enabled version from the sender.
For team workflows where annotations matter, use a tool with shared review — annotations live in a comments thread rather than the PDF file. The PDF itself can stay locked while discussion happens externally.
Prevent it next time
Don't restrict comments on PDFs you're sharing for review. When receiving a PDF you need to comment on, check restrictions before starting and ask the sender for an unrestricted version if needed.
FAQ
Why is the comment tool greyed out in my PDF reader?
The PDF has annotation restrictions. Either unlock with the owner password, request an unrestricted version, or use Flint which flattens annotations into the page (working around the restriction).
Will my flattened annotations be editable by the recipient?
No. Flattened annotations are part of the page content. The recipient sees them clearly but can't move or remove them. That's the trade for working around restrictions.
Can I add comments without the file owner knowing?
Flattened annotations are visible to anyone who opens the file. There's no covert annotation. If you need private notes, keep them outside the PDF.
Do annotations affect file size?
Minimally. Text annotations add kilobytes; large drawings or many highlights might add more but rarely meaningfully. Flattening can sometimes reduce overall size by simplifying the structure.
Annotation locks lift in Flint — flatten on save and your comments stick to every copy.