Takeoff Studio Pro
When a new issue of a drawing arrives, lay it over the old one - old in red, new in green, linework on both in grey. Every change is boxed and listed, and you can cloud it in one click. Then swap the new issue in and your measurements carry across.

In the PDF tab choose Overlay New Issue and pick the new PDF, JPG or PNG. The page you're on shows in red, the new issue in green, and linework on both in grey.
Drawings on the same paper size are laid on top of each other as they are. If they don't line up, choose Align by 2 points and click the same point on each, twice; 3 points also corrects a stretched scan.
Changed areas are boxed and listed as added, removed or changed. Click one to zoom to it. Detail sets how small a difference counts: Fine finds small text changes, Coarse ignores them.
Cloud adds a revision cloud round a change, or Cloud all changes does every one. Clouds are notes, so they stay on the drawing and in marked-up PDFs.
Replace drawing with this revision makes it the project's drawing. Your markups carry across, changed pages are reported, and markups on removed pages are kept aside, not deleted.
Help: Revision compare has every option in detail.
Part of Takeoff Studio Pro. Measuring in Takeoff Studio is free. Pro is £24.99 a month or £199.99 a year, the first month is free, and you can cancel any time from your Microsoft account.
Replace Drawing is free: it tells you which pages changed and keeps markups on pages that have gone aside rather than deleting them. The overlay, the change list and markup baselines are part of Takeoff Studio Pro.
Yes. Line it up with Align by 2 points, or 3 points if the scan is stretched. Detail Coarse ignores small differences such as scan noise.
Yes, in the overlay panel. Use Both, Old only and New only to switch between them, and Overlay strength to fade back to the drawing.
Yes. Overlay Another Page compares two pages of the same PDF.
Capture a markup baseline before you replace the drawing, and you can see which measurements changed afterwards with Select New and Select Changed.

Click inside a room and it's traced to the walls. Auto walls measures walls on CAD drawings.

Floor, ceiling, wall and skirting quantities for every room, with totals per finish and level.

One measurement becomes every item it needs. Then an Excel bill in NRM2 order with live formulas.

Box one socket or light and every match is found and ringed for you to check.

Voids out of areas, joists and studs at centres, repeated rooms and pitched roofs.

Scale from the title block, scale regions, the drawing register and text search.
Measuring is free. Takeoff Studio Pro is £24.99 a month or £199.99 a year, and the first month is free.