This app was mentioned in 28 comments, with an average of 2.54 upvotes
It's an Samsung app. If it's not on your phone (anymore), just install it from Play Store (Link). You can open PDF files with this app and use all inking stuff which you know from the Note 8. Have fun :)
Write on PDF if you have a Samsung device.
Or this one from Adobe
I am speculating here, it might have something to do with the fact that an S21U does not come with an S-Pen. I had the S21U for about a year, I do not remember whether I had it or not on my phone.
Here is the link to the play store "write on PDF":
https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote&hl=en\_US&gl=US
https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote&hl=en_US
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Simple, easy to use, never had a problem and free.
https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote&hl=en_US thats the app and its preinstalled firmware.
I used https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote&hl=en_GB to sign my mortgage papers a couple of months ago on my note 3. It did the job for me but I haven't used it since.
Samsung makes an app for this https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
Official Samsung app: https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
I have a note 9 and use this https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
I would use the App Write on pdf directly from Samsung
https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
Actually this one works like what you need. https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
Samsung's "Write on PDF" has the best pen integration (imo).
https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
Draw on PDF is decent but split screen sounds like the way to go.
If it's not already on your phone, install Write on PDF. It's a Samsung app that allows you to draw on documents.
It came preinstalled on mine too, I don't know if this link will work:
https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote
If not, I'm sure you can try googling the APK
You can download Write on PDF by Samsung to write on PDFs
+1 been looking for a better alternative for a long while. I don't want TW, but I want write on PDF.. https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote&hl=en_US
Write on pdf seem a good solution. https://play.google.com/store/apps/details?id=com.samsung.android.spdfnote