Would it still be tough to prove the RO for PR if I find a job at a Canadian company that is 100% remote and I work from US?
That would make it nearly if not absolutely
impossible to prove you qualify for the credit.
One of the requirements for work outside Canada to qualify for the credit is that it be pursuant to a
TEMPORARY ASSIGNMENT, noting that both of these terms are important elements; that is, to qualify for the credit requires that the PR has been
ASSIGNED to work abroad by the Canadian employer (not just allowed to work abroad for example), and that
the assignment is temporary in the sense that the PR has a position working IN Canada which they will be returning to, to that position IN Canada, when the temporary assignment abroad is completed.
The information in the manual/guide referenced and linked by
@scylla is a good resource, but even applying that is still TRICKY. It cannot be emphasized enough: if you do the homework, and do it objectively, and you are still considering relying on this credit to meet the RO, it would be foolish to proceed to rely on it without getting a dispositive, definitive opinion from a reputable Canadian immigration lawyer, based on a paid-for consultation in which the lawyer reviews the actual details.
Note, if the setup is oriented toward getting RO credit for time living abroad, one way or another the odds are it is NOT going to work. IRCC and CBSA and the IAD have adopted a very strict approach to this credit which very much limits its application. It is clearly intended to avoid penalizing PRs who are more or less compelled to go abroad on lengthy assignments by their Canadian employer. Not to facilitate living outside Canada.
That is, this credit is intended to be for a PR who is living and working in Canada, and THEN a Canadian employer wants the PR to do work abroad, on a temporary assignment basis (and thus is not a transfer to a position located abroad).
This credit is NOT intended to be for PRs who want work that will facilitate their living abroad.
This is not the standard, not the rule, but IRCC has clearly adopted policies and practices toward applying this credit in this way. That is, they have adopted rules and standards which when applied, strictly as they are, that is what is accomplished.
I cannot say for sure, but it seems rather obvious that if a PR is working abroad in their home country or generally located abroad in a particular area, especially one where the PR was living before becoming a PR, claiming a
working-abroad-for-Canadian-business credit is almost certainly going to get examined under a powerful microscope with much skepticism.
Small and family run businesses, and very much so businesses established by the PR, are especially scrutinized.
The Problem: And it is indeed a big problem . . . the problem is that there is no way to get a ruling ahead of time. The risk that IRCC will not allow the credit is totally on the PR. And the only way to find out is to make a PR card or PR TD application relying on the credit, or attendant a Port-of-Entry 44(1) Report review after being determined to be in breach of the RO. The only safety net, and it is not a good one, is H&C relief.
To be clear: this is not to say no one actually gets this credit.
Many do. There are many scenarios in which PRs are SENT abroad by their Canadian employer and the PR needs the credit to meet the RO. And they are allowed this credit.
But IRCC makes a concerted effort to keep this from being a loophole for PRs who are not actively settling in Canada PERMANENTLY.