The revision letter
A supplier issues revision D of a component specification. The transmittal says "updated per customer feedback". Somewhere in forty pages a tolerance widened, a material grade changed, or a test was removed. Accepting the revision without knowing which is how nonconforming parts get approved.
The supplier is rarely hiding anything. Their engineer made the change they were asked to make, and two others that seemed minor, and the transmittal describes the first one. The job on the receiving side is to find the other two.
A worked example
Say revision C of a bracket specification calls for a hole position of 42.0 ± 0.1 mm, material to a stated aluminium grade, and a salt spray test of 500 hours. Revision D arrives with the note "updated per customer feedback", which refers to a change in surface finish you asked for.
Comparing C and D shows the finish change, as expected. It also shows the tolerance on the hole position widened to ± 0.2 mm, and the salt spray requirement reduced to 240 hours. The tolerance sits in a dimension table on page 9. The test change is one number in a paragraph on page 31.
Neither is necessarily wrong. The supplier may have good reasons. But they are engineering decisions that belong to your side, and accepting revision D on the strength of the transmittal would have made them by default.
What the comparison shows
Upload revision C and revision D. The change list shows each altered requirement with its page number in both versions and the old and new text. Tables of dimensions and tolerances are compared cell by cell, and tables that did not change are held out of the result entirely. Repeated headers, footers and document numbers are excluded, so the new revision letter in the footer is not reported on every page.
Paragraphs that moved between sections are reported as moves rather than as a deletion and an insertion. Two-column layouts, common in supplier datasheets, are read in the right order.
What to do when something changed
Not every change needs a fight. A sensible triage is to sort the change list into three groups: changes that match what you asked for, changes that are editorial (a corrected reference, a reworded sentence with the same meaning), and changes that alter a requirement. Only the third group needs an engineer's decision.
For each change in that group, either accept it and record why, or send it back with the page reference from the change list. A supplier who receives "page 9, hole position tolerance changed from ± 0.1 to ± 0.2, not agreed" can answer in one email. A supplier who receives "we have some concerns about revision D" cannot.
Why the transmittal is not enough
Most supplier quality procedures already require a review of incoming revisions. The weak point is what the review is measured against. If the reviewer reads the transmittal, then reads the sections it mentions, the review is only as complete as the transmittal. Comparing the documents first turns the review around: the reviewer starts from a complete list of what changed and works through it, instead of starting from what the supplier remembered to mention.
It also makes the review auditable. The change list and redlined PDF show exactly what was in front of the reviewer, which is a better record than a signature on a cover sheet.
Building it into release
- Compare every incoming revision against the last accepted one before it enters the PLM.
- Store the change list and the redlined PDF with the revision as the record of what was reviewed.
- Use the API to trigger the comparison automatically on document receipt and route any substantive change to the responsible engineer.
- For high-volume suppliers, agree that each revision is sent with your comparison output attached on the next round, so both sides are looking at the same list.
Drawings
CAD-exported PDF drawing sheets carry notes, dimensions and title-block text, and those are compared. A dimension that changed from 42.0 to 42.5 is found. Changes to line work alone are not text and are outside the scope of a text comparison, so a drawing revision still needs a visual check alongside the change list.
Watch the file size on large drawing packs: the web tool takes up to 20 MB per file and 25 MB per pair.
Getting started
Specifications up to 30 pages can be compared on the homepage without an account. A Business plan covers 500 comparisons per 30 days on documents up to 100 pages, which suits a supplier quality team, and Enterprise adds API access and private deployment. The manufacturing and automotive industry pages have more on the workflow.
Try it on your own documents. Upload two versions on the homepage. Documents up to 30 pages need no account, and nothing is stored.
Industry page: PDF comparison for manufacturing