In Germanic languages, the p sound in Proto-Indo-European became f. I have wondered if the p sound means that the word does not come from a Germanic source. This is because words that have p in them usually come from Latin. Does the presence of that letter mean that the word is not Germanic?
Grimm's Law predicts that Proto-Indo-European *b would turn into Proto-Germanic *p. However, Proto-Indo-European *b is vanishingly rare, and some scholars argue it didn't actually exist in the oldest reconstructable forms of the language (only appearing later). Regardless, though, an ancestral *b is probably the source of a few native Germanic words like English "apple", cognate with Russian jabloko and Gaulish abalom.
Grimm's Law also had certain exceptions where it didn't apply. The most common of these is after *s, which gives us native Germanic words like English "spew", cognate with Latin spuō, or "sprout", (probably) cognate with Ancient Greek speírō.
No, because PIE *p does not always become f. It does not in the cluster sp, for example "spin" < *spen, "sprawl" < *sper. Germanic p regularly derives from b, e.g. "deep" < *\dheub. Germanic *swompuz "swamp; fungus" is attested in all branches of Germanic as well as Greek σομφός: the reconstruction *su̯omb(h)o-s is a bit of a problem because of the variability in aspiration. Nevertheless, it's clear that those instances of "p" are in Germanic words. Other examples are "apple" and possible "peg" < *bak
Even in the case that the /p-/ is word initial, there are some words of Germanic origin containing it due to some irregularities mainly. As an example I quote the word Patzer (mainly known as a term for a bad chess player via Yiddish, but a more general word in High German) that is related to the regular High German word Batzen "heap, pile, lump", the initial P is a typical Upper German (Bavarian and Alemannic) dialectal feature. Packen "paket" is another example, in this case related to backen "to bake".