Adobe Acrobat (desktop): open the Comment panel on the right side, select the sticky note tool, and click anywhere on the page. A note icon appears and a text box opens for your comment. To edit a note later, double-click the icon. Acrobat is also one of the clearest options when you know you may need to export comments or print with annotations later.
Preview on Mac: go to View, then Show Markup Toolbar, and click the speech bubble icon. Click a spot on the page to place the note. Preview saves annotations directly into the PDF file, so the notes usually travel with the document when you share or reopen it elsewhere.
iPhone and iPad: open the PDF in Files or a PDF reader like GoodNotes or Notability. Tap the annotation tool (a pencil icon), then choose the note or comment option. On iOS the note attaches to the page and stays visible when you reopen the file, which makes it a good route when the PDF needs to move between phone, tablet, and desktop.
Android: Adobe Acrobat Reader for Android and Xodo both support sticky notes. Tap the edit or comment icon in the toolbar, then tap where you want the note on the page. If you plan to share the annotated PDF, check whether the app is saving the comments back into the file instead of keeping them only inside the app session.
Google Drive: open the PDF in Drive's built-in viewer, click the comment icon in the top bar, and draw a selection around the text you want to annotate. Google Drive comments are stored separately from the file and are not embedded in the PDF itself, so they are better for collaboration than for a PDF you expect to download and review elsewhere.